Tbh it's not like he'd be living any differently to how he does now. He still eats the shit he used to when he was single after Kat likely refused to stop cooking for a manchild after doing a full days work, he still sits alone in his office for most of the day, he still watches amazon and tweets shit at 2am, he's still the same narcissist from a year ago.
The only thing that changed was he'd just drop a mention of her as though he had to reinforce that he does infact have a life outside his office. A trophy to say that he is actually not a complete failure because he's in an adult relationship. Not that he knows what it means.
When she goes, likely all that's going to change is the whiny tweets will get more frequent and his streams will get longer.
Going to jump off this point and add my two cents regarding feeling bad for Phil. There are some cows on here that I think genuinely deserve pity because they're largely cows because of other influencing factors in their life, and sometimes we even see signs of them trying to change only to be dragged back down by enablers, or even sometimes just a lack of support. But, for me, one of the reasons I enjoy Phil as a cow is there's nothing to feel bad about.
If Phil genuinely cared about Kat, it would be a whole other thing. But let's be real, he doesn't. As shown above, Phil's life really didn't change after he got a girlfriend. Kat was a trophy girlfriend and nothing more. Phil was happy because he could trot her out to brag about winning the Kentucky Derby. If he loses Kat, he isn't upset because he lost a partner who he genuinely loved and spent time with. He's upset because, in his mind, it is a blow to his pride. It's like the gout mansion. Does Phil need all that space? No, but in his mind, it is a status symbol. Look how successful he is, he's in this condo that is bigger than his previous home. He's doing what adults do. What else do adults do? They date and get married. Well, Phil is a successful adult (in his mind) so by his logic, he needs someone to date and marry, otherwise he is failing.
I admit, there were times after the Leanna saga I felt bad for Phil, notably around this time last year when he was talking about not having friends and being lonely. But then I just thought about Phil's old friends in CT, and how he sold them out because of the Project 7 nonsense. Phil says he wants friends, but what he really means is followers. He wants people who look up to him and respect him, who offer him praise, who put him first etc.
Anyway, with Kat, he didn't want her as an actual girlfriend. It doesn't sound like they have a lot in common or really went out and did anything special. With other people, you could make an argument for not knowing what they do in their offtime, but with Phil...eh, not really. Phil is pretty open and transparent about what happens in his life. Again, see the post above, most of Phil's time is accounted for, and we have tweets to fill in the rest. I sincerely doubt after his streaming schedule he went out with Kat or spent any time whatsoever trying to personally connect with her. At most, they might have watched a few Netflix shows/movies together. But, can you really imagine Phil having any deep/meaningful moments with Kat? I feel like their stream together awhile back really summed up their relationship. People would ask her questions, she'd have nothing to say, and Phil would try and vaguely fill in the blanks with offhanded things he remembered about her. They weren't particularly complex questions, either. Imagine if the situation was reversed and someone asked you those questions about your gf/bf. You'd be able to pretty confidently answer them because, unlike Phil, you actually give a damn about the person you're with.
Being in a relationship takes effort, and you have to genuinely care about the other person. Phil is a lazy narc. He doesn't want to be someone's boyfriend, he just wants a status symbol and someone to cook for him. If it weren't for his weird pigroach pride, he'd be way happier taking some of his paypig money and hiring an actual maid.