As for keeping 100,000, this seems unlikely as if you're in care the local council isn't going to take over payments whilst you still have funds in the bank.
The £100,000 includes the value of your home and any other assets - I read it as a way of staying in your home to receive care or to receive care in a care home and for the remaining spouse to stay in teh home until they died. The Govt would presumably take security over the estate down to a value of £100,000 which remaining relatives would inherit on the death of both OAPs.
So the old rules wouldn't apply anymore once your asset base, including your home, reaches £100,000 (ie. 4x higher than it is at the moment). I read it that even after paying for social care, you would have £100,000 of assets remaining.
In 2013 the number of homeowners with no mortgage exceeded the number with a mortgage for the first time. Most of these people are older. Younger people are struggling to get on the housing ladder. It's been labelled inter generational inequality. Therefore this proposal asks the older generation to use some of the housing market gains they've been lucky enough to enjoy to fund their own care down to a value of £100,000 for the remaining assets, rather than the state to pay and their relatives receive an inheritance well in excess of £100,000 - which for those lucky enough to inherit continues to perpetuate the inequality.
I think the problem is that there is no cap on care costs - it should be for example that if you have sufficient assets you pay for costs between £100,000 and say £500,000. This would encourage insurance companies to develop products that you could buy to insure yourself should your care costs exceed £500,000 in my example. I also think it seems illogical that 'physical' illnesses like cancer are covered by the NHS but neurological illnesses like Alzheimers are not.
An interesting summary from December last year about the challenge of social care:
www.economist.com/news/britain/21711928-sticking-plaster-solution-system-needs-quadruple-bypass-taxes-will-rise-fund
A summary of how they pay for social care in Japan
www.theguardian.com/social-care-network/2014/mar/27/japan-solution-providing-care-ageing-population
What we can all agree on I think is that it is a complex problem that is only going to get bigger over time with the ageing population, longer life expectancy and advances in medicine.