It's not about 'illness' necessarily, but just because you were lucky enough not to have had a c-section or physical issues arising from your labours doesn't mean a lot of other women don't. True - some who do have to cope with other children out of necessity, but the majority of women will have at least some help. Also most sensible women will not be scrubbing the house within two weeks of giving birth - they will most likely be doing the bare minimum.
Any woman who doesn't have help around the house and has to wrangle a toddler and new born alone so soon after birth, would likely not have any support if she were to return to work either, so as well as working full time she would still have to wrangle the baby, care for the toddler and do the housework - difficult at the best of times, doing it with a new born after the physical marathon of pregnancy and sleepless nights would be nigh on impossible.
Also, the toddler may be kicking you while you BF, but at least you can BF - pretty impossible to do while stuck in an office with your new born at home with DH / nursery / with a CM.
There is also the emotional aspect to consider. I would argue that most women relish the chance to bond with the baby that they have grown for nine months and for the majority, parting from them so soon would be a massive wrench that would put many at risk of PND.
Even though physically, I could have returned after two weeks, it would have meant giving up breast feeding and would have destroyed me emotionally. I would have also been beyond knackered as DD is a poor sleeper and DH does a lot of driving, so I would have had to take care of most of the night wakings.
It is great for you that you managed to go back after two weeks and seem to be happy with your choice, but I think you need to appreciate how highly unusual it is that you were happy to do that.
I think the 2015 changes are a great compromise - it allows fathers to take some time off with the infant while protecting the mothers rights to spend a decent amount of time with their newborn should they wish.
I'd even like to see them go further so that any time after the first 6 weeks could be transferred to the father if that's what would work for the family as long as safeguards were put in place to ensure that abusive men did not make their wives go back to work simply so that they could benefit from the time off.
Also existing rights of the full 12 months to maternity / paternity / parental leave should not be eroded.
The problem with the original idea in the OP is two-fold:
Firstly it erodes current maternity rights due to the non-transferability idea - suddenly the right of a women to spend 12 months with their child if that is what works for them and their family is reduced to 7 months.
Secondly the idea that any woman who is still physically recovering from the birth after the first month should be transferred to sick leave is massively problematic because sick leave has a lot less employment protection than maternity leave.
I'm all for equal parenting and giving families more opportunity to choose which parent, if either, is home with the kids. The current maternity / paternity / parental rights should be changed and extended to allow this, BUT those changes should not in anyway be to the detriment to the hard won rights that women currently have to spend the first 12 months with the infant that they have carried and given birth to should they wish and their family circumstances allow.