How Rafa sees it and how it actually is, aren't the same thing. His chances of reaching 25 Slams are almost nil. First of all, if he really does return to Slam-winning form, that will only inspire Novak to work harder. Secondly, the talent is gradually percolating up - this is no longer a soft top 10, with Medvedev, plus Tsitsipas and Zverev, Rublev and Berretini, and then the Young Brats (FAA, Sinner, Alcaraz) showing strong development. Not to mention Thiem wanting to re-assert himself, and all of the other dangerous players who could play spoiler.
Next Gen hasn't yet shown that it is a premier generation, but it is a helluva lot better than Lost Gen, and they're also entering their prime years as Novak and Rafa are in their mid-30s, not late 20s like they were when Raonic, Dimitrov etc were trying to break through. Meaning, peak Next Gen is not only (a lot) better than peak Lost Gen, but the old guys are older, so the gap is smaller. And the gap may actually have vanished, or that is what Medvedev hoped to have proven.
That said, maybe Rafa has one more push in him to win a couple more. But that's it, and even so the pathway is quite narrow. No way he wins five more.