If you have to think about cross-shard transactions then you have to think about all the things on your first list too, as they are complexities related to transaction. I fail to see how that could possibly be simpler.
Cross-shard transactions are only a tiny fraction of transactions — if the complexities of dealing with that is constrained to some transactions instead of all of them, you're saving yourself a lot of headaches.
Actually, I'd argue a lot of apps can do entirely without cross-shard transactions! (eg. sharding by B2B orgs)
