You and I will be dead by the time the Bitcoin supply is totally mined. So it's not something you need to worry about right now.
Bitcoin will not be difficult to buy in the future, but if you target the amount of 1 bitcoin it will definitely be difficult. Even 10 sat could be worth a lot in the future. An answer that will be difficult to ascertain, because it takes a long time to spend 21 million bitcoins and we may not even be around to witness it. I don't really think that far, as long as the market provides bitcoin to buy then the only choice is to buy any amount even if it can only afford $10.
Maybe 1 bitcoin will be worth a billion dollars by then, if it gets that scarce. It is definitely not going to stay at 1 million dollars like some other people here are saying. Of course, we haven't even realized a 1 million dollars price yet but there's over a century left until this happens.