I've never stopped being surprised how many people prefer Red Dead's multiplayer, or GTA IV's, to GTA Online. While Online has many issues (especially the shark card economy, though I would argue that is almost a necessary evil unless we're all willing to pay subscription fees to play), I think it's mostly a large improvement over both of its predecessors. I played GTA IV multiplayer for a week or so and gave up. I played Red Dead multi for a couple weeks, then retreated to single player and played that over again. In both cases the central problem was clear to me: there was just nothing to do. Both had a few fun game modes but free roam - which has always been the core of the Rockstar open world experience to me - was a barren wasteland with just a few activities (like hideouts), where players mostly resorted to killing each other over and over again. GTA Online is even prone to that a lot, but there are tons more activities - especially now, after the addition of VIP and CEO content, and free roam challenges. And when you DO just go killing each other, there are myriad ways to do it: Pegasus vehicles and phone abilities and muggers and mercs and all that. Red Dead and GTA IV tended to be the same damn thing repeated a thousand times over: spawn, start shooting (or getting shot), wash, rinse, repeat.
I don't know how much a hypothetical Red Dead Online would resemble GTA Online. But despite the problems I've had with GTA Online, I am absolutely hoping that they use it as a jumping-off point for the next Red Dead game. If it's just the same stripped-down, empty multiplayer as before, I'll spend exactly as much time with it - which is very little. Whatever else you think about GTA Online, over time it has come to be absolutely packed with things to do, both in and out of free roam.
Regarding the idea that there will never even be another Red Dead (or Rockstar-made) single player game, we are getting way ahead of ourselves. WAY ahead. Yes, GTA Online has been a cash cow and they have milked the hell out of it. But let's not forget a few basic facts:
1. GTA V famously made $2 billion before GTA Online even launched. Nothing to sneeze at, there.
2. Red Dead Redemption also made a bundle and was massively critically acclaimed, and was almost entirely a single player game; the multiplayer portion was an afterthought for most players I knew. (A bunch of my friends played that game; few of them did anything with multiplayer other than mess around for an afternoon.)
3. Dan Houser was a main writer on Red Dead and the GTA series. He considers himself more a writer than a developer, is the rumor about him, and The Benz' lawsuit explicitly says that the Housers contributed more on the writing side while he was the technology guy, and also the guy who wanted to (and did) design Online. They just ousted The Benz and have regained sole control of the company. I find it hard to imagine that they're going to give up the thing they seem to love best and steer entirely in a direction pioneered by a guy they just publicly fell out with.
I do think Red Dead Redemption 2/Red Dead 3 (whatever it ends up really being called) will have a robust multiplayer akin to GTA Online - maybe not quite as frequently updated because the player base would likely be smaller. But I don't think for a second that Rockstar is ready to abandon single player. If we're looking mainly at the lack of GTA V DLC as our evidence for that, we only have the GTA community to blame there; the GTA IV DLCs were well known to be financial under-performers, and I don't think Undead Nightmare was quite the windfall they wanted, either (even though it ruled). So we will very probably get a Red Dead sequel in single player form, but as for a DLC for that game - maybe not.
EDIT: It is absolutely hilarious that this forum auto-replaces the name "Leslie Benz.ies" with The Benz.