The same way they’ve done it with every previous O/S they’ve stopped supporting it. They… stop supporting it.
They can’t make old computers and software don’t disappear or evaporate (although they’d like to if they could). Those old computers and software still exist but MS won’t lift a finger to keep them updated or help anyone with them.
MS is perfectly fine with losing some existing users who can’t afford to upgrade or replace their PC’s, because the amount of money they make from people who do upgrade far outweighs any money they lose from people who don’t/can’t.