Now that Alcaraz is invincible at hardcourt slams, I'm certain he will win around 30 slams at the pace he's going.
Especially because of that match vs. Zverev in Australia whereby Alcaraz started moving like a 45-year-old and still pushed Zverev to tiebreakers.
So we know that even when Alcaraz loses his mobility in old age he'll still be brilliant!
Other threats will come. They always do. Top players get scouted and analysed to the nanometer. Up & coming players sets the bar higher. Sampras had few threats until Fed. Fed had few threats until Nadal, then Djoko. Djoko had little threats after Fed & Nadal got old and he was still young enough until Sinner, Alcaraz. Injury, motivation, all those are threats too.