Depends on whether the rate of salt extraction is precisely equal to the inflow over the long term. The Great Salt Lake is extremely salty and basically only two species can live in it: brine shrimp and brine flies. With about the corresponding level of "tourism" you'd expect.
And that is with industrial levels of salt harvesting. Would we expect the salt harvesting of these inland seas to be equal to or greater than the daily inflow of salt via the sea water? Seems dubious to me.
On the other hand, there are still many other benefits. Migratory birds love the Great Salt Lake. The Great Salt Lake Effect[0] on precipitation in this desert seems unambiguously positive.
If you make it a bay of the Mediterranean, it should behave no differently than the rest of that ocean.
With the various pipeline/tunnel/channel options that are admittedly more realistic, at least at first, there might very well be the kind of problems you mention.
I think most of the oceans are a barren wasteland.
That said, as a shallow and, at least at first, nutrient rich part of the ocean it could be a flourishing part of it. But if it’s filled via underground pipe, it’ll be isolated from sources of ocean life.
I think most of the oceans are a barren wasteland.
This is as far as I know, incorrect. I'm not sure why you think it would be true. Oceans, aside from areas like the Dead Sea are some of the most vibrant and life filled ecosystems we have. Even at the most crushing depths far from the sun there are entire ecosystems on the ocean floor.
The GP is technically correct. By volume much of the ocean is practically a wasteland because of the oxygen minimum zone [1]. A few animals like vampire squid are adapted to this zone but for the most part, the vast majority of ocean life is found in the top and bottom most layers.
In my mind, these new seas would have a steady influx of salt water, but only sweet water would evaporate out of them. And since I don't see the mechanism by which salt would get sequestered or removed from these bassins, I'd also guest their salt concentration would rise quite quickly.
Of course, it might still be interesting to have a huge water surface in the area that effectively serves as a rain generator, but it might not be the same thriving ecosystem we imagine when we say "ocean".
Filling basins with ocean water will not increase the volume of sea water. It will probably increase the area of sea water (thus increasing evaporation, causing more precipitation, and speeding up the water cycle, which has a net cooling effect on the planet), and it will lower sea levels. Win win win win!
Is there any reason I'm missing why expanding the ocean won't just give us more ocean?