I do think they'll be a reduction, but I think more like 25-30%. I just can't see 50% happening.
My STs (I have two) cost £1426 each, so a 25% reduction would bring them down to £1069.50, which is still a hefty sum. Matchday tickets in my block are £101, £80 and £66 so they would come down to £75.75, £60 and £49.50, although I don't see how even they could justify keeping the Cat A/B/C system.
At present the Ticket Exchange credits me £75 per ticket sold (1/19th of ST price) regardless of category, but in the Championship (23 home games) that would be £62 if ST prices remained the same or £46.50 if there was a 25% reduction in ST prices. I think that the Exchange would hardly ever be in operation though, with most if not all matches going to General Sale, so that's another issue for ST holders who wish to sell tickets for a few games (I can't say I have any sympathy for those that cherry-pick the biggest games and offload the rest, which in previous seasons was pretty easy, but even now is difficult, with several games already going to General Sale and thousands on sale even if the TE does open). That alone will surely discourage a lot of ST holders from renewing, especially the touts.
Regarding the stadium, I can't see them closing the top tier. Maybe some of the food outlets or bars on level 5 wouldn't open, but shifting those that do intend to renew to the back of the lower tier or behind one of the goals would piss off even more long-time ST holders (including me).
One benefit of not being in the PL or Europe is that I will be cancelling TNT and saving £30 per month. Actually I'll cancel it even if we stay up; I'll only miss one or two away games per season and can live with that.