I think you mean there are a billion Windows users (not TT users) and only half have moved to Win 10 (globally - US is about a third on Win 10).
Turbotax desktop only has 4million users including Mac (25%?) and in the US the Win 10 share has flipped from 2/3 to 1/3 over the course of 2025. Theoretically that leaves 1 million Win 10 users wanting Turbotax (4 x 0.75 x 0.33) but these are just averages so it's probably lower given Intuit's decision announced several months ago. The rest will either deal with Win 11, try out online, or switch to other s/w.
Intuit's made it increasingly clear their direction is towards higher revenue online & live advice services, and made several offers to desktop and ItsDeductible users for discounted online services.
So yes if you don't want to upgrade to Win 11 yet or use online (or set up Win 11 VM to run TT), you'd need to switch to HRB or TaxAct.
Good luck.
