It would have been the same mistake as "Enterprise". It wouldn't have killed Trek but it would have lead to the same result as "Enterprise".
There was nothing wrong with the idea of making Voyager and the show had excellent characters. Voyager was the logical step after TNG and DS9.
The problem with Voyager was that it was somewhat sloppy handled by the writers and producers. They had the chance to come up with something really good but they never used the potential the series and the characters had. Maybe they also should have waited a year or two before launching Voyager it came too close on TNG and DS9.
Voyager wasn't bad. It was a good series. But some things couldd have been better.
Making a retro series like "Enterprise" after Voyager was the big mistake and a Sulu series would have been even worse. They should have continued in the 24th century. I mean, they had created that wonderful universe with all those interesting species, developed what was started in TOS and expanded it and what did they do then? They abandon it all and go back to a TOS state oreven worse a pre-TOS state with only Klingons, Romulans and such.
It would have been like the record business should have abandoned all the modern technology and gone back to mono recordings and 78 rpm records which crashes if you drop them. A Sulu series would have had the same result.
TOS was a wonderful series with wonderful characters. They should let it rest in peace instead of those futile attempts to milk the last possible money out of iit.