My take on wish enhancing (with the understanding, I've never played on Medium Lifespan before):
Most LTW are between 30k-40k, right? Assuming the sim has Ambitious, and a LTW that is normally 40k, that would bump them up to 45k, right? (Hurray napkin math!) Wish enhancing would double it. So they complete their LTW, and now have 90k. That's still another 70k they have to pick up somewhere, and most of those LTW aren't usually attainable in a week or two, under normal circumstances. Maybe I just have really terrible strategies.
Players will also want to be careful about life extensions, since they'll need 4 of those large tombstones in order to change worlds. That's both founders, heirs 1 and 2. And as Metro said, speed is the important part. So does anyone really want to be stuck in Sunset Valley, waiting for heir #2 to finish another life extension span to get those final LTHP, since we can't kill 'em prematurely?
Maybe I'm just making too much out of it, but I usually can get the large tombstone on Normal. Medium, I don't know, you're essentially halving their lives. Am I just being overly melodramatic?
Edit: Also, if we choose Isla Paradiso as a world, are we required to stick with land-based homes, or can we move into houseboats?