I might be remembering this wrong, but IIRC any hero of a battle pass will be available to everyone either one or two battle passes after they launched.
And I doubt this will be a massive issue. The game already has plenty of heroes in all roles so it’s not like new players have slim pickings. Hell, Dead by Daylight is still going strong six years later despite virtually all post launch survivors and killers being in individual chapter DLCs you have to purchase if you want all of them.
I heard some rumors, but haven't read anything about heroes turning free after a few seasons.
All I could find when googling was this quote:
"If you happen to skip a season and miss out on a new hero, Blizzard did say there will be ways to earn heroes that were released in previous seasons. You'll be able to earn them by completing special challenges or by acquiring them directly through the in-game shop with Overwatch Coins."
Now we don't know how difficult or how much time those "special challenges" will be, but given the amount of premium currency you can earn per week, I'd say it's safe to say it won't be as easy as reaching battlepass lvl 55.
To clarify, I don't think the game will die and when I say "break" I mean it will lose popularity drastically, not die out.