Almost all Monero will be mined in 3 years.
"Almost all" is misleading (it sounds to me like 99% or something, and down to just a maintenance reward or none at all). The actual number I've seen is something like 85% in 4 years, which means even if 4+ years there will be significant mining. The current proposal for a maintenance reward doesn't kick in for around 10 years or so.
So if initial distribution in mining matters. It matters more now in early adoption when the release schedule is trying to drive adoption than it does in 3 years
That depends on the future value, which is impossible to know. Bitcoin rewards were cut in half, yet the value of a block (and of all mining) now is vastly higher than it was 2+ years ago.
In fact another way to look at the schedule of mining rewards would be to attach a current value at the time they are mined. By that measurement Bitcoin is
much less than 60% mined or whatever the current number is assuming constant-value BTC.
I am worried if the transaction fee is not high enough in 4 years time, will miner still serve the network?