Well, there's obvious problems with online, starting with that it's technically illegal and unregulated in most US states, which is a pretty big one. Secondly, the rise of solvers and other AI have greatly increased the risk of your opponents using them to beat you (RTA aka real-time assistance). Besides that, many other forms of software assistance such as HUDs and database datamining were pretty much always accepted so if you don't want to do those things you're at a disadvantage. There's also serious risk of collusion , team play, card sharing, doubly so on unregulated sites.
It's hard to overstate the difference in skill between online and live. You see VERY weak players buying into games with $5k stacks live while online games with $200 buyins are considered very tough. Even regular players in high stakes live games make surprisingly fundamental mistakes, like checking back extremely strong hands on the river rather than betting them for value, or almost never bluffing.
I would say that if someone plays poker 20+ hours a week and has a losing year live they are also probably just a losing player. I'm friends with many live poker pros, and I've never heard of any of them having losing years. Obviously they pick games they know they can beat and mostly stick to them.
Tournament variance is extremely high and I think being a live tournament pro is unnecessarily risky unless you sell / swap action which most pros do. Or pray you run good and win one. But cash is a safer bet.
As far as spending all your time in a casino in order to make a living, that's basically the equivalent of spending all your time in an office, it's a job and you go to it. It certainly can be grindy. I understand it's not for everyone. But it's not super far removed from many other in-person jobs. And as others have said, it can actually be nicer than staring at a screen 10 hours a day, especially staring at a screen to play poker, because it has the social aspect with the other players and the dealers, the tactile aspect of the chips and the cards, etc.
I'm not advocating for anyone to become a live poker pro, certainly I'm sharing my project on hacker news with the hope that some "serious recreational" players take interest in it since very few poker pros lurk here. Most people should not play pro poker for various reasons - on Hacker News the obvious one being they can almost certainly make more money and easier money with plain software engineering, software engineering is generally more intellectually engaging, and also most people aren't mentally equipped to deal with the swings.
I'm mostly doing it currently because it's a niche that I understand well having done it in the past, and I wanted to do an indie software project and "make money playing poker to reduce my burn rate while building a poker blog and training app" has a natural synergy to it as an entry point into bootstrapped software entrepreneurship. Plus so far its my best received project, has a small but growing audience and respectable retention on the app despite it being an MVP.
Moreso than advocating people become live poker pros, I'm just noting that basically anyone who wants to can win a bunch of money at poker, if they're willing to study the massive amount of resources that exist now. Those resources always existed but the advent of solvers has changed those resources from "these are very good heuristics beating the games" to "this is the solution to the game under certain constraints".
Based on experience talking to "hacker news" type people, they tend to be introverted and so would strongly prefer the nature of online poker rather than casino poker, but I would again warn people that online games are both far tougher and require a lot of extra tooling and precautions that aren't necessary in a casino game.
Plus, at some point, in live poker, you get to put all the solver nonsense away and just look a man in the eye and decide if he's bluffing you or not :)