Some people the 5 starter system gives the 2 best stages to: Diddy, Falco, DDD, and ICs. Thinking about this now, I don't understand this since any player is in control of 3 stages. Going into any set they can guarantee themselves one of three stages. This means at best they can only get their 3rd best stage... But like lets look at things.
Switching to 9 stages adds 4 more. Every player gets 4 strikes so they are in control of 5 stages, guaranteeing one of them.
In the 5 starter system guaranteeing one of 3 stages, the following characters can guarantee themselves:
Diddy: FD, SV, YI (I'm not sure if YI or BF is better for him, but I can see YI being a tad more beneficial overall)
Falco: FD, SV, BF
ICs: FD, SV, YI
DDD: FD, SV, YI
Now with the 9 starter system guaranteeing 5 stages:
Diddy: FD, SV, YI, PS1, BF
Falco: FD, SV, BF, CS, PS1
ICs: FD, SV, YI, PS1, CS
DDD: FD, SV, CS, PS1, YI
Now remember that each opponent has 2 strikes so can strike the best 2 stages of your character, so at best you're only going to get your third best stage.
For all the stages above I listed the stages in order of how good it is, decreasingly of course. I might be off a little bit, but it's close enough. By this I mean in the 9 starter system Falco's best stage is FD and his 5th best is PS1.
And similar to only getting your 3rd best stage in a 5 starter system, you can only get your 5th best stage in the 9 starter system.
So now ask yourself (the reader), is YI that much better than BF for Diddy?
Is BF that much better than PS1 for Falco?
Is YI that much better than CS for ICs?
Is YI that much better than.... YI for DDD? lol
The answer to me is no, not really. The change between their 3rd best stage in the 5 system and their 5th best stage in the 9 system doesn't differ too much. The only one that changes noticeably is ICs, but tbh it's a stretch and not too much of a change really. DDD doesn't change at all. Falco is about the same-ish imo. Diddy is about the same-ish imo, and it is the exact same if you think BF is his 3rd best stage in the 5 system instead of BF like I figured.
So really, as people suggest the flaw in the 5 starter system and prefers the 9 system over it, nothing really changes.
So yea, the 9 system won't prevent the quality of those 4 characters having over powered stages in the 5 system. It just so happens that those 4 characters thrive on "neutralish" stages. You would need a 11 or 13 starter system before you really saw a noticeable change in their game 1 viability.