The problem with this theory is that, unless the money is coming from somewhere else, it hinges on Phil being honest, responsible, and disciplined enough to actually give his mother the money to put away into an account for him. If he had any of those traits, why would he need his mother?
Phil says that his mother suggested filing for bankruptcy. He's also prone to throwing people under the bus, even preemptively, so that may be what was really going on there.
Ultimately, I think it says enough about how Phil truly feels about his parents the fact that he moved as far away from them as he possibly could without leaving the states, and makes no effort to see them on his own dime and own time. I don't doubt that they try to help him, but it seems extremely unlikely to me that Phil is anywhere near cooperative enough for a savings account being contributed to by his tips from retards to be a thing. I mean, for it to work, he would have to be the type to admit that he has a problem being responsible with money, right? How else could his mother talk him into handing money over to squirrel away?
Obviously it's more than plausible for Phil to be the grifting bastard that he is while secretly having enough money to put into a savings account, but that doesn't explain the WWE Champions account of his that's more than $40k deep, that he's terrified of having linked to him. And REALLY visibly hates that we found. Could he be faking it in order for us to think that he's doomed, when in reality he's set to pull the biggest W over us mentally-ill detractors with a secret savings that's a lot deeper than NotDSP's gacha investment? That WOULD be a hell of a kick in the dick. No doubt about it.
But I don't think so, because that's way more foresight than can be believed from the guy we know has made certain mistakes without even knowing what he was doing. Not to mention, for that to be an act, Phil would have be a good actor, and we know for a fact that he isn't.
I'm sticking with the most logical, predictable explanation of Phil's anticipation of his own future: that he doesn't think about it at all. Like I've said before, this is not a man who has experienced legit financial uncertainty, and that has clearly resulted in him being lax about things he shouldn't. In every respect, Phil takes full advantage when fate enables his bad traits, and he does it without a second's consideration. If he worried about his whales leaving him, for example, why would he mistreat them so? That's not a risk I can picture from a man with concern for his own future.
As for Phil banking on an inheritance, I suppose, but if he is, it's not the reason he seems unconcerned with his retirement. He just expects fortune to smile upon him every so often, because financially, it does indeed keep doing it.
TL;DR - The pigroach has way too much established lore that makes it obvious how much he just bumbles his way through life because he keeps surviving it with a six figure income intact. Having the cooperation of handing his mom money every month doesn't fit that at all.