If mainstream adoption is the key, having a maximum of 21 million units seems problematic. You could argue that the price would exponentially increase to match the demand to the point 21 million will be enough when considering the price per coin, but that still doesn't necessarily solve the problem.
Focusing on 21 million BTC units isn't correct.
At this point most people can already not afford a whole Bitcoin, so it makes much more sense to deal with satoshi units. I have been getting used to dealing with the satoshi unit a lot due to the lightning clients I use. It's so convenient and easy to use, and more importantly, people prefer to own whatever number of whole satoshi units over 0.0000xxx part of a whole BTC unit.
If you think satoshi units are low value units, then you will be surprised to find out that one single satoshi can be split into 1000 milli satoshis on lightning.
https://en.bitcoin.it/wiki/Satoshi_(unit)