It wouldn't disincentivise people with a £1m + pension pot, because they're already above the current £276k lump sum threshold. Ie they'll already pay that tax on any additional money they're paying into their pension.
It affects people with a pension pot between £400k and £1million, taking an extra 5% of that tranche of pension savings for most people, and up to an extra 10% for the wealthiest pensioners. (Like I say, >£1million, and they're already paying it)
There's still an incentive to pay into pensions due to deferred tax, but it is a significant hit.
It might encourage people to take their tax-free lump sum early. Not necessarily retire - although since you're the restricted in how much you can put into pension after you've taken any of it, there might be a short-term disincentive for pension saving and work for people approaching retirement.
The main difficulty is for people due to retire soon who were relying on the lump sum to pay off their mortgage, especially since mortgages don't usually go past age 65.
I think the government would need to secure agreement from the mortgage industry that they would introduce mortgages on small amounts (up to £30k - that's the max most people will lose to tax) past age 65, based on pension.
It would be an outrageous consequence if the lack of time to plan meant people had to take out a taxed lump sum since they're too old to get a mortgage extension and hence were forced into a higher rate of tax for that year! That really would destroy confidence in mortgages.
Reducing the tax-free lump sum would be painful (I would personally lose loads) but it would be much, much less harmful than their other crazy ideas like fixed-rate tax relief.
The tax free lump sum is genuinely a tax break: this is income which has never been subject to income tax. And reducing the allowance is progressive, without distortions, and without significant disincentives to pension-saving or - worse - disincentives to work.
It needs to come alongside pension provider changes, to reduce impact.
And also, if everyone is being asked to put more into the state, it needs to come alongside some serious changes to reinstate incentives to earn and to save. It's crazy that pension credit is almost as much as the state pension, for example. Working more and saving more should always benefit you. That's key to the social contract.