Would have been a much better Jon moving up in his mid to late 20s. The 37 year old version we saw against grandpa Stippy looked slow, less athletic and timid at times. His fight IQ is still elite but physically he's clearly declining.
However, he would have been moving into a much deeper HW division. Cain, JDS, Werdum, (coming up on prime) Stipe, Reem, Barnett, Mir.
I don't think many others would have given Jones too many problems, but there were 5 solid guys who could have given him issues.
These days there's only Aspinall. Ngannou could have been a problem too but Jon tactically waited for him to be out of the picture. There's a reason he's avoiding Aspinall like the plague. He's the one guy from todays division that could have been champ even when contending against some of those all time great HWs.