Re: 51% problem still existant after all coins have been mined ?
Some will keep mining for tx fees. Some will keep mining to keep bitcoin alive. Some will mine for fun.
And some will have made so much money through BTC, that they would leave a portion of their legacy to pay for mining.
That's an interesting idea. A person with 1 million BTC (you know who you are) could perhaps run a script that submits a transaction with a 0.1 BTC fee every 10 minutes. It could subsidize miners for 190 years.