If all Bumrah can manage is 300 Test wickets (or just a few more) in this day and age, that is telling you that he has issues which should preclude him from unanimous consideration as the GOAT bowler. How many career Test wickets do you think Marshall, McGrath and Hadlee would be able to get these days? A lot more than 300 each and that means they would probably be of a lot more value to their teams because they would also be getting those wickets quite cheaply.
I would have a hard time ranking Bumrah ahead of Hadlee no matter what he does in the future (unless some amazing transformation occurs). Equal? - maybe, but ahead? - not likely. Hadlee was able to sustain a massive workload for a very long time while bowling at a very high level which is something that Bumrah is not even remotely able to do and that is very unlikely to change in the future. Those attributes of Hadlee's are not something minor but rather, something very important and Bumrah is lacking those attributes. So even if Bumrah's quality of bowling is slightly better, he will be missing important attributes which Hadlee had in abundance. How can Bumrah possibly be the undisputed GOAT bowler with such deficiencies? Do you think that a bowler who's a glass cannon compared to other bowlers is better than those other bowlers when those other bowlers are far more robust and can also bowl at an exceedingly high level? Don't you think those other bowlers would be able to contribute more to their teams long-term than Bumrah and don't you think that's important in evaluating the greatness of a bowler?
Here's Hadlee's record for his last 12 years 5 months against non-minnow competition (before this, he was an amateur cricketer; when Bumrah started playing Test cricket, he had already been a professional cricketer for about five years, having joined the IPL in 2013):
View attachment 45080
Workload for 12 years 5 months: 155 balls/innings! Worked to death and still averaging 20.9 against top teams.
Availability: incl. SL: 69 Tests/75 = 92%, excluding SL: 63/69 = 91.3% (high availability will help his team)
Now Bumrah last year when he was starting to fall apart at the seams towards the end:
View attachment 45081
Workload for ONE year: 82 balls/innings.
Availability so far: 45/71 = 63.4%