Let's be real. Aston Villa can get the money from Grealish transfer but who will they sign? It's clearly not an attraction zone. High profile players don't want to play there. They know they can't get any cups with Villa. You can only get unambitious Tier B players with money. I don't think Aston Villa have a plan to be in Top 6 teams in Premier League anyway. They will have another season ending in 10th-14th spots.
They'll just get the money and rethink their financials. They don't need to replace him, they can live without him as they're not a top tier team. Every year they survive in EPL, never showing much consistency. Presidency's happy to have made a huge profit that will get internalized. As you were saying I highly doubt they'll find a decent substitute for Grealish.
They do not have to replace him directly, they can replace him with 3-4 different players. I remember from "moneyball" movie (and that was based on real life) they had a great hitter, but he got bought by some other team, so they were down a great player but only one, what did they do? They replaced 3 worse players with 3 better players to cover the difference.
So let's assume Grealish goes, and they get 100 million for it, they could spend 25 million for 4 different players and improve 4 different places, 1 will be place Grealish played so that will be worse but 3 others will be better than what they have now, so that 3 total getting better could "hopefully" will cover that one worse.