Well that's the thing, you
can use mBTC if you want to. And I suspect most newcomers will. It's just human nature to form a habit and stick with it, sometimes. So really, this all comes down to personal preference. If we had all been using mBTC from the outset, then that's probably what we'd still be doing. Chances are, most people who get involved going forwards will feel more naturally inclined to use either mBTC or μBTC. Once that becomes the norm, the rest of us will probably start using it too.
There's a question related to this thread but I'm quite confused about the answer i.e. about the 8 decimal places rule.
What if in 20 years thanks to a big increase of BTC valued they will not be enough (let's dream a 20,000,000 $/BTC or even more

) ?
Lurking around this forum I've sensed it's not possible to add new decimal places, is that right ?
For the base layer, you can change just about anything if there's consensus for it. The tricky part is reaching that stage. Basically, it requires a fork.
For anything built on top the base layer, such as Lightning, it's much easier. Lightning can already deal with units smaller than satoshis.