I got to thinking more about this and I wondered two things:
1) Maybe hydro and solar can "wear" with each expedition. so you get to keep them, but only for 3 turns or something until you have to repurchase. Perhaps that keeps the challenge but makes it feel more fair.
2) It feels like the pylons are actually the most powerful upgrades because they can be placed anywhere. So I wondered if solar and hydro should be cheaper (only giving access to pylons later). Because I found myself placing pylons willy-nilly early, where instead it's better to place them away from water.
Oh, I forgot to mention in my first comment that seeing your old dead rovers was such a nice touch!