From that standpoint, the Browns are far better off plugging away and trying to build the team around him.
So it begs the question, why would the Browns make this adjustment to Garrett's contract if they don't then intend to trade him? There are a couple plausible scenarios. One was already mentioned, which is it allows teams to potentially make such a ridiculous offer, the Browns have to listen. It would also theoretically allow his agent to search for trades. Maybe there's peace of mind there.
Realistically, it likely just acts as insurance against their own potential incompetence and terrible fortune. Should the Browns not be able to capitalize on this year's draft, they don't move the team forward, it gives them the ability to reassess the situation and perhaps move on from Garrett. It makes that a year to year proposition.
There may come a point where they don't feel the team has progressed enough and they do then think trading Garrett makes sense. It just doesn't make much sense for that time to be now, especially after going to the trouble of going through the contract fight just a year ago.
The Browns are better than they were a year ago. The incentive for them is to see if they build on it, both by continuing to add talent in the draft and finding out if perhaps Shedeur Sanders can become an effective starting quarterback. If that were to happen, the Browns might be a playoff team this year.
However unlikely anyone feels that Sanders can be that quarterback, imagine trading Garrett only to find out that Sanders was good enough to lead this team to the playoffs only to fall short because the defense is compromised without Garrett. As great as those draft picks might be someday, they can't help the team now.
Did I mention that Garrett has a 'no trade clause'? There's that too.
There may come a time when trading Myles Garrett makes sense, but it's not now.