Why would they not move to PS5 if they will get 18 of the best games they didn't play yet for a single plus subscription they might already have made?
"I'm short on money and buy games on bargain, but I'm moving to Series X and ignoring those games that could keep me in check without spending for games for around 2 years, other than some release I really want, instead."
Sony is basically locking the majority of new Plus buyers on their ecosystem, the PS5 got a huge value boost for those people.
Late adopters aren't likely to spend money on a subscription in the first place. But among those who are willing to pay for a subscription, you have to remember that Game Pass is a thing and can boast with a much larger selection of titles to play. Additionally, Xbox Series S is in consideration for people who are short on money, not just because of its price tag, but also the financing option to pay small monthly rates.
This isn't as clear cut as you want it to be. It would be shocking if Sony hadn't already run several predictive models to determine whether a PS4 price cut is worth it or not. They had great Black Friday deals for a few years in a row and probably concluded that not enough of those cheap PS4s resulted in long term subscribers for PS+. In other words, going cheap doesn't lock people into their ecosystem.
Generally speaking, I have my doubts that locking people into an ecosystem works to begin with. The vast majority of people that are retained are most likely still there because of future games, so they wouldn't have picked another console either way. I know the idea is that "you've spent hundreds of dollars in this ecosystem, so would you really be willing to leave all that behind and choose another console", but the big flaws with that are, for one, buying a new console doesn't make your old system and games go away, and two, for a lot of people the value of their previous purchases degrades over time, so they don't mind moving on to something different that they consider better.
A generation ago there were Xbox fans who believed that Microsoft had locked gamers at large into their ecosystem with digital purchase histories and all the gamertags with their achievements, but it quickly turned out that that wasn't the case because way too many 360 owners opted for a PS4 as their next console. The PS5 will have to succeed on its merits, rather than the PS4 being assumed to have laid a foundation where a PS5 is the only logical choice after a PS4. What Sony has shown with the PS5 is solid enough to transition gamers from the PS4 to the PS5 or bring in gamers who didn't own a PS4, so I doubt they'll see any need to be aggressive with PS4 pricing on the final stretch where they'd lose money on individual consumers in hopes that the investment will pay off later with a PS5 purchase.
Of course Sony runs lots of prediction models.
Pay in mind we are talking about a person that bought the PS4 because of the price cut, they are already there, the incentive to jump ship for the XSeries because of Gamepass is now mitigated by the PS Plus Collection, as they are already ellegible for that.
They choose the PS4 and will gets an amazing selling of those games whenever they jump to the PS5 automatically if they bought the plus.
Almost 1/2 of the total install base of PS4 have plus, if you consider people that sold their system it should be more than 1/2 of the actual users, so the changes anyone will buy plus is high, specially knowing the benefits from having it on next gen now.