1kBTC right now is worth more than 300k$
For many people is more than their lifetime savings. Do you think they trust bitcoin enough to put all their savings in it ?
I wouldn't.
Its in the question, the definition of retirement savings would be you are putting most of your capital into that. Its not a good idea to own just one thing expecting it to grow but the OP question was this, how much to retire
The falling block reward is something of a time bomb. That means it has to rise in value to sustain large scale, and thus secure, mining or process many more transactions to make up for it. It's either success or death and staying in the few hundred dollar range in the far future equals doom.
I thought I read that it was also about rising transactions per block, this makes sense as often a working capitalist system is efficiency and mass production not large size alone justifying a system