Just assuming, I can't look behind the scenes and do not have a fitting professional experience to make more educated guesses:
If they will start tackling MP (even if they may recruit for it now) in serious fashion only later down the line when most SP content has already been finished or delivered, then I don't fully buy the "diverting resources from SP" argument. Or rather, the argument becomes weaker in my eyes. Hiring an MP focused dev won't touch the SP side of things much, either, if we assume them to be working around the expected capacity.
But I suppose we'll see more down the line and are better able to judge this beyond guesses going either way, pro or not.
Lastly, my personal opinion is that too many give take "GTA:Online" in their mouth (or put it on the screen, rather) with a negative connotation.
I played and play GTA:O on occasion, I know what people likely refer to and that is how gameplay can end up there. I also agree that if CP was to go into that direction, it would need to tweak or change certain things to result in a 'more mature' end result. But the general core concept of GTA:Online isn't bad, it's rather something players benefit from in my eyes. Why?
- Just like in the SP version you use the same big open world or map - you are not detached from the world
- You can freely make a or multiple characters
- You can pursue countless missions and activities, including social ones
- You can purchase private or business related property and many types of vehicles
- Any sort of interaction possible with strangers in this big open world, from friendly / ignorant to hostile
- Gambling (yay)
- Mission editors, custom content players can add to a certain level
- And likely more I can't think of right now
The design itself is good. If CDPR would aim for something similar in concept, I think the trick is taking the good aspects and tweaking what people consider bad aspects. If it ends up less arcade-y I'd also consider adding VOIP functionality to speak to people. If it ends up in some sort of dedicated server basis players can further specify or customize, you can even see the potential of different communities beginning to form that offer different content on their servers, like PVP or RP oriented ones.
But alas, this all is speculation or preference posting. In the end we can only wait ... and / or suggest things to CDPR. Or, well, discuss our own preferences or dislikes amongst each other.