what i think is that you have a point but it's to early for your suggestion and mbtc might cause more confusion than anything, when bitcoin will reach 10k or 100k mbtc will be the standard for many that can't afford a whole bitcoin, but for now i would use BTC as a suffix which is fine
Do you know that bitcoin is limited by 21 million number?
In the case of mass adoption bitcoin will have to be shared on mBTC or even for satoshies to be enough to everyone to use. But for that bitcoin will have to cost a lot. And only rich people will have capital in bitcoins, and others use only satoshies.
Bitcoin will never reach 21 million of their supply and it's mean the supply will less than the final accumulation by the maker of bitcoin. mbtc is really difficult to say. bitcoin is better to say than mbtc.
21 million will be reached in 2140 when miners will end their block reward, why you say it can't be reached? you are spreading false info