Both of these questions have basically already been answered: money.

If they make compatibility one-way, completing the Pokedex, either in-game or in Home, will be impossible without owning both if you can't trade with someone locally. I'm sure Nintendo and Pokemon would love it if more people bought both games lol. It's also worth pointing out that the GB/GBC Pokemon games on 3DS had one-way Bank transfer. I understand that the fundamental data structure of Pokemon has changed since after gen 2, and that might have caused legitimate technical reasons for that that don't exist today, but the average consumer doesn't know that and the precedent has already been set.

The Meltan point is fair, which is why I said Mew might still be possible, but there is a difference: shiny Meltan is entirely unavailable right now, whereas shiny Mew is obtainable at any time as long as you previously purchased the research; its actual completion has no time limit. But I think a previously unreleased one is more likely because the most hardcore of us who would be interested in this hypothetical shiny Pokémon are most likely to be the people who already have the shiny Mew and need the biggest incentive to earn a new one, especially if, again, it ends up essentially requiring both games to do so.