It might make Valve unlikely to make another game and a lot of people, including myself, would be hugely disappointed but I don't think it would have any major negative effects beyond those two.
I didn't even know seedless watermelons were a thing, so it saddens me to discover that they are, and that they're bad, since the main reason I don't like watermelons is having to deal with the seeds.
The more competition Sims has, the better. Maybe EA will actually try and innovate... HAHA. I've been keeping an eye on Paralives and Life By You, and there are a couple others coming as well, I think. It'll be a good time to be a life sim fan in the next couple years.
As others have said, you don't need to play the old ones, and I doubt Sony will bring them to PC, so your best bet is probably emulation if you really wanna play them.
Pretty fucking good question, actually.