I think a lot hinges on who is driving the decision for one partner to become a SAHP, who primarily benefits from that etc.
In some cases, it will genuinely be a joint decision that benefits the family as a whole. For example, if one partner has limited skills/qualifications and very low earning potential, then the overall household income might be lower if they have to pay out for childcare costs than it would be if that lower earner became a SAHP. Similarly, if the couple have dc with disabilities or significant additional needs, the couple might conclude that it isn't feasible for both of them to hold down a regular job, so one parent might agree to sacrifice their career for the sake of the family as a whole. And in some cases, it may just be a lifestyle preference shared by both partners who decide that they are happier to live on a lower income because this enables them to simplify their lives, reduce workloads etc.
In other cases, the decision to have a SAHP is driven primarily by one person, typically the SAHP, and the set-up is accepted (or in some cases, just about tolerated!) by the WOHP, rather than being actively wanted or needed. I know quite a lot of families like this, but I don't know any where the decision to have a SAHP has been driven primarily by the WOHP.
I must say the majority of WOHPs that I know would prefer not to be the sole breadwinner and to carry the huge pressure that that brings. For all that it might be mildly appealing to have all of the domestic chores taken care of for you, most people don't live in mansions these days and there is only so much housework that actually needs to be done, so sharing it between two WOHPs isn't really that big a deal. And even less so with all of the modern technology that is available, not to mention the ease of getting a cleaner in for a couple of hours each week if you feel the need. Also, contrary to what some might think, I find that most people - men and women- do actually want to be involved in raising their kids, rather than outsourcing it all to their spouse/partner. As the main breadwinner in our family, I'd have really hated DH to be a SAHP. We did it for a short while after moving house, and while he certainly worked hard to make things "fair", we both recognised that he had the much easier the if the of the deal. Thankfully, he hated it as much as I did because he missed the social interaction of work, and the intellectual stimulation, so that phase didn't last!!
I guess what I'm saying, ultimately, is that what's fair in terms of a SAHP returning to work depends partly on how the decision for them to stop work was arrived at in the first place. If it was genuinely a mutual decision, then I agree that there needs to be mutual agreement and negotiation about the timescales and manner in which the SAHP should return. If the decision was driven primarily by the SAHP, with the WOHP having agreed to facilitate the SAHP's choices but not actively wanting the arrangement, then I think it's totally fair enough for the WOHP to say that they don't want to facilitate that set-up any more.
And of course, the WOHP cannot expect the SAHP to return to FT work and carry on doing all of the domestic stuff. That would clearly be unfair. However, there might be some circumstances in which a SAHP returns to work on a very PT basis, with the kids in school all day, leaving them plenty of time still in the working day to carry on doing most of the domestic stuff. Context is everything here, but as a general rule, I think it's reasonable to expect that both partners should have a comparable amount of leisure time.