Never really thought about this thing.......... How will we be able to use Bitcoin if there will be like 50 million Bitcoin users? It'd be a shame if Bitcoin won't be able to sustain so many peoples' transactions.... It's growing fast!! Hope we will be able to use it just like we are right now!
With onchain blockspace resource limited, the more Bitcoin users, the less often they can make onchain transaction. Some basic math: with 1 MB blocks or about 250 000 daily onchain transactions possible mean: for 2M userbase 1 onchain transaction weekly for everybody, for 8M userbase 1 onchain transaction monthly for everybody and for 50M userbase 1 onchain transaction every half year for everybody. These are averages, if some doing onchain transactions more often than averages, others need to do onchain transactions less often than averages.