.858 is pretty good, but point well taken.
Someone else mentioned the comp to Aaron, and of course, there's only one Hammerin' Hank, but I think there are a lot of similarities.
Hank won 3 GGs early on, stole over 20 bases 6 times and had power, too. His first 5 full seasons, he hit 27, 26, 44, 30 and 39 HRs (ending at age 25).
Starting at age 32 he hit 44, 39, 29, 44, 38, 47, 34, 40 (465 PA at age 39), 20 (382 at age 40 .832 OPS) and 12 at age 41 (543 PAs).
His OPS never dipped below.852 from age 21 to 39, when he hit .832. More amazingly, he only hit under .900 twice in that period: .895 at age 32 .852 at age 34.
In no way am I trying to say Betts might duplicate Hank's career, but Hank showed it can be done. His body proportions changed over his career, but his power actually improved at age 32:
24-31: 288 HRs in 5346 PAs/4786 ABs .962 OPS
32-39: 315 HRs in 4853 PAs/4208 ABs .954 OPS
First 12 years: 398 HRs .943 OPS in 7855 PAs
Last 11 years: 357 HRs .909 OPS in 6086 PAs
Betts is way below Hank in HRs by age 26. Betts has 31, 24, 32 and 29 his last 4 years. He has 116 HRs over the last 4 years (2762 PAs) and a .917 OPS.
He will almost certainly not come close to Hank's numbers, but he may have a long career with little loss of power, if not an increase in power.