Okay , take it as 6.3 trillion , assuming there are all 21m BTC mined.
That will still be 300k , which is much lesser than necessary to cause a problem resulting in need to further divide Bitcoin.

It is unlikely that Bitcoin will ever cross a million $ , but I expect it to definitely hit the 10k range.
People who expect it to reach 1m in a few years are extreme fanatics.

Comparing Bitcoins future growth to the 300x boost it saw ages ago is stupid.
If Satoshi told 2 people about BTC when he first envisioned it and tripled the Bitcoin enthusiasts in a minute , we should not expect that to happen again in a minute.

TLDR; Bitcoin will rise , but even if it becomes as big as USD is now , it will be under 40k$ per Bitcoin (of todays USD value)
We will not need to further divide BTC in a long long time and if we do , we will have far more capable devs waiting to do so.