Whether the rolling stock tilts or not has probably been already decided. And I'm fairly sure rolling stock isn't transferred to HS2, only services. The current services on WCML are just going to stop more. And HS2 has already got the bids from manufacturers submitted for new trains, confirming that transfer of rolling stock from WCML to HS2 is unlikely.
You're right, the 221 replacement class for the WCML is probably already decided. My point was, this stock will either be tilt capable or it won't be, and that's probably decided now as you pointed out. But now we have a slight potential, given it might be very slight, that HS2 could be cancelled.
Now if this 221 replacement stock is non-tilt, it will be far less useful for the next 20 years on the classic lines if HS2 is cancelled and the WCML remains the main route and journey times have to be fast.
I'm not at all suggesting the new bi-modes would need to run over HS2. As you point out there will be new trains procured for that if it goes ahead.
My point is, the only logical reason why non-tilting stock would be ordered for the classic WCML is if HS2 was definitely going ahead, meaning tilt wouldn't be important after 2026 when most long distance services use HS2, where journey times will be more critical.
I.e. HS2 phase one to go ahead = tilt not so important on the classic WCML anymore as high-speed headline journey time services will use HS2.
HS2 phase one not going ahead = WCML to keep the headline journey Intercity services and so tilt to remain very important to keep headline journey times low.