lasrry if you 'shared' roles then you have no idea sorry. You would have an idea if you actually had been in full cahrge of said baby and toddler for several days in a row, whilst been expected to maintain all the standards of washing, ironing, cleaning, cooking to the same standard than a SAHP would have. Do that for a few years. Then, only then, you will be able to say you have an idea (and even then, know that you won't because you will get the breaks at work, the mental stimulation and the opportunity to talk to adults!)
A baby and a toddler, if they are 2 or 3 years apart isn't 'just a short time'. It's 6 b*y years! Would you be happy to do a job that is so demanding for 6 years and get no recognition whatsoever? Would be happy to then have someone coming over to tell you that 'your job isn't up to standards'? Serioulsy?
Because I don't know anyone who would be staying in that job if it was the case tbh. (And actually many women do bet divorced because they are so taken for granted. Being divorced, at least, they DO get a break every so often...)
As for having help nearby with grandparents... well I'm happy that YOU do have some support but bear in mind this is far from the case for everyone. Some people have family nearby who they don't get on/don't want to help etc... Some people have no family nearby (When I look around me, this the case for most people tbh).
As for your 'not national wage' again, I'm happy to see that you are financially wealthy bit this is far from the case that only people who earn well have a SAHP. Lots of people on low revenue do. By default because they can't pay for the childcare :(
In effect you are talking about a few select few with non average situation where everything is so easy that they can have breaks, help at home, grandparents AND enough money for a holiday etc...
Maybe stop thinking about that very small group of people and look at the bigger picture.