Quite a few pundits are saying the expulsion is an excessive punishment, but I think they're missing the point. The reason spying is banned is because it gives the spies an unfair advantage. Fair enough if it was in the first few weeks of the season, and a team was low/mid table, then maybe a fine would suffice. But in the context of a play-off, and a chance of promotion, the stakes are much higher. Given that they've admitted spying before, it casts doubt on their entire season, particularly given the incredible turnaround they had after sacking Will Still. If they weren't expelled, the EFL would effectively be saying we know you cheated, but now that you're over the finish line, we're going to let you have the gold medal anyway!
With any crime, the punishment has to act as a deterrent. If there's no meaningful deterrent, people will just commit crimes!