It's interesting what you say, and yes it will be fascinating to see what happens in the coming years.
I think the situation is a bit more complex. Schools have withdrawn, but it's mostly Preps, smaller senior schools (more likely to be girls' schools and those outside the south-east, where pupil numbers are much lower). HMC represents the larger, historic schools which were historically boys schools and either still are or are now co-ed and those schools have been keen to remain in the TPS. It is used as a selling point to attract teachers (and you'd be surprised by how hard even very top schools can often find it to have a big pool to choose from in expensive cost of living areas) and also a selling point to parents. The larger schools are making clear it doesn't mean rising fees, but it does mean quality teachers.
One of the reasons why some schools withdrew early, was they feared further rises to employer contributions. The pension revaluation which means employer contributions can change, is increasingly looking like the 2024 implementation won't see increased charges for employers. So, schools which have absorbed the costs and think it's worth it to maintain staff, should be okay for the next few years and another evaluation beyond that one will be a good few years off. There should be more certainty for schools.
I agree that there could be a tipping point. At some point, perhaps after the next 2 revaluations, if costs to employers rise again, more could leave and after a certain point, many more might go, feeling there is no advantage in staying in. Some elite schools might stay as a way to make themselves especially attractive. I don't think we are at that point yet, nor as close to it as you think. The big names all remain within it. The top tier schools of HMC remain within it.
Any school pulling out is bound to emphasise numbers leaving and play down recruitment issues. However it's worth bearing in mind that there's a massive recruitment crisis across teaching. Fewer people are training and people are leaving both the independent and state sector. Often the independent sector isn't seen as a much better option - lots of independents pay less than state schools (you'd be surprised how many) and some of the features of weekend work and longer school days with extra curricular mean people are leaving or not keen to switch from state. Again, parents might be told there's a huge pool of applicants for every job, but in much of the south-east it isn't the case, partly because of the huge cost of living.
It's a tricky one for parents to judge. Teacher pensions sound expensive and parents understandably don't want fees to rise too much. Of course schools have lots of choices to make about where they spend their fees all the time - new buildings, other facilities and staff. There are choices within each, and staff will always be a key resource. Parents pay their fees expecting good teachers. Longer term, poorer pensions will make recruitment harder still at the start of teacher careers. Teaching pays reasonably but isn't well paid compared to many careers, but a reason people put up with that, is their pension is worth 1/3 of their entire rewards package - if that's removed, the whole thing becomes less attractive. Those who might have trained with an eye to an Independnet school career might choose something else. There's enough bad press about the job already to mean that even if the time of a downturn in the economy, when teacher recruitment usually rises, even less people applied to train. Schools do worry about filling vacancies with high quality people, whatever they might say to parents. Even those that are top 20 and pay well, often don't have many applicants.
Anyway, we will see. It could be that in a number of years the climate will be quite different and the schools will all or mostly be pulling out. I don't think it's imminent and that means that for a good while yet, it really will become a 2-tier system of the attractive schools within the pension and the less attractive that have left. Perhaps you're right and that won't last too long. However, the ones that left first or are leaving now will be impacted in their staffing. It might not be easy for parents to spot it or to be able to see the consequences - what else could they compare to - everyone only knows the experience they are getting. But if you were one of the first to withdraw, perhaps 4-5 years down the line, with the steady, even if low churn of staff, and you might see quite a different staff in terms of qualifications and quality. That's something schools have to assess isn't it I suppose - are they happy to pick from a smaller pool of applicants? Will it impact the education they offer? Will parents notice? Will it impact their place in league tables etc.
As a parent I would certainly be interested to know if schools I was considering sending my children to remained in ir had left. I wouldn't just see it as a direct correlation to fees, but also be thinking about the impact on my children in their final years of schooling, in terms of the staff that might be there then as a consequence.
Perhaps you're right and suddenly there will be a mass exodus. I think it will need an external event to prompt that. Another big increase in employer contributions could well have that effect. That's not on the cards at the moment. Schools are no doubt watching carefully and seeing what's happening. I do t think we're as near the tipping point as you think. Look into exactly which schools and which type have gone. You'll see the market is polarising. There are increasingly the bug successful schools, and those struggling. That's been happening for sometime and actually lots of Preps and smaller schools (especially girls schools or those outside south east) have merged or closed. The pension costs pushed some that were clinging on over the edge. But there are also schools which have always had big numbers and are even more popular since Covid made people think fees might be more worth it. They want to retain their competitive edge. The TPS is quite a key way to do this.