As mentioned above there is no block limit. Fee problems won't be a problem for a very long time as we are reaching 17,000,000 Bitcoins mined today.
I fail to see what the total number of bitcoins in circulation has to do with fee problems... Could you explain the link here?
It requires so long time so do not worry about that. In theory our Sun will also be shut down but nobody worries about that
I think the more suitable question - what to do with lost BTC (like lost private keys and etc)
https://en.bitcoin.it/wiki/Controlled_supply We're currently in reward era 3 (we're about ~2 years into this era).
The block reward will be 0 at the end of era 33...
So, a raw estimation would be that the block reward becomes 0 satoshi in ~120 years... At this point, all ~21 million BTC will be in circulation and miners will be rewarded with the sum of the transaction fees in the blocks they mine.
This is long before our sun shuts down (AFAIK).
Or am i missing something here...
The second part of your answer is actually quite interesting. Personally, i don't think that lost private keys are a problem, the more people lose their keys, the less BTC is actually being traded, the higher the price for the btc in my wallet will be
At the moment, the smallest denomination is 1 satoshi. This is 0.00000001 BTC. However, theoretically, if all but 1 BTC would be lost it would still not be be a problem... We could just change the smallest denomination to 1 millisatoshi (or something) so you could send 0.00000000001 BTC to somebody if you wanted to. At this point, the theoretical BTC price would probably be > 150 billion USD/BTC, so my 1 microsatoshi would still be the equivalent of $15 if you traded your microsatoshi for FIAT.