The reality of Melee without Marth would benefit the spacies, especially on stages like Final Destination because Marth overall has great ground dominance and an excellent punish game. Marth's chaingrabs are especially deadly against the spacies on Final Destination since the large amount of space and lack of platforms on Final Destination exposes the fact that the spacies lack options to escape the chaingrabs, and it doesn't help the fact that their weight makes them combo food to the entire cast. It would definitely help brush off a matchup for the spacies, and especially other top characters who lose the neutral game to Marth, and characters considered to have a 50:50 match up against him, like Captain Falcon.
Not only would it help the spacies, but it will also help many characters especially the mid-tier and low-tier characters who do not have the tools and options to keep up with Marth's speed and range near the ground. At high levels of play, these mid-tier and low-tier characters have a difficult time to punish and create openings, especially since they can't utilize poor matchup knowledge from the Marth player, and bad execution...well sometimes. Even though it brushes off one bad matchup, there are still many other matchups that they have to focus on since Marth isn't the only bad matchup for them.
As a Doctor Mario main, no Marth in Melee would definitely help alleviate a bad matchup, especially since Doc does poorly against characters who outrange him.