Singleplayer is too short 69 missions...some of those lasting under a minute (Masks)
Many of them also involve multiple stages and character switching. It's short only if you rush it.
Only 4 bounties and a handful of random activities that don't respawn.
That is a small number, yes. I've never actually done a bounty but don't you have to search for them like treasure hunting in RDR? That would be why if I'm right.
Why do random events that don't respawn need to respawn?
sale vehicles online where in singleplayer you can't. Limited garage space and why no inventory for singleplayer?
I think the reason you can't sell vehicles, rob stores/ATMs and other things like that in the SP is simply because that makes it way, way too easy to make money.
You need to actually do things to make money in SP. In Online, these things are there for convenient, quick cash when you need it. And you do need it. You can't enter passive mode unless you have $100, for example.
I really don't understand why limited garage space is a problem, it makes you want to only save the cars you really value, and there's no inventory because why would you have an inventory? What would you need it for?
Singleplayer story is also very sloppy
there is only a handful with very little payoff
Did you miss the part where
R* has nothing but multiplayer "Packs" planned for multiplayer.
There are a few "major" paid DLC packages coming and I'm fairly sure it's not going to be solely for Online. I could be wrong, but I very highly doubt they're not going to do anything with the SP.