The idea that a founding world leaving might undermine the Federation's cohesion, or make it easier for other worlds to lose faith and pull out, is certainly the concern, I think, and if I recall that's at least implicitly why President Bacco was so frantic about the decision. Holding onto Andor does seem to have been viewed as more important than keeping a non-founding world in the fold, so there must still exist some sense that the union of the original five members is the foundation of the Federation. Symbolically, as you say, if nothing else. How much weight and significance that idea truly has within the Federation is an interesting question; as
rfmcdpei points out, the UFP has grown extensively over the last two centuries and Andor's actual participation and practical importance had been greatly reduced by the 24th century. Then again, as
Sci mentioned upthread, the Federation's own politics doesn't appear to have caught up - the founders still hold permanant seats on the Security Council, for one thing (as an aside, I wonder what happens there with Andor gone. Will another member - Rigel, say - be promoted to join the remaining four or will it now be four permanant positions?). So I think the argument that Andor leaving sets a dangerous precedent is a valid one, and is being treated as such in-universe.
However, I don't think we should assume that Andor's leaving is necessarily as major a blow as might be assumed - again, both in-universe and out-. Indeed, if the Federation is so unstable that Andor's leaving threatens to undermine it to any serious degree than we might question what the point of the Federation was in the first place. Why should 154 civilizations linked by trade agreements, mutual defence and cultural exchange lose sight of what their organization is about just because one of the older members, who's been having problems for some time anyway, decides to withdraw? And why should we as readers lose faith just because it leaves? Are we saying that the Federation is only a worthwhile protagonist nation if it never loses members? Even if there were only 2 planets left, if they were keeping the ideas and outlooks of the UFP alive, surely it would be worth it?
The chapter in
Paths of Disharmony where Andor made its decision known ended with President Bacco's newly reaffirmed commitment to holding the Federation together, about there being "much work to do" and that she'd "start now". So, again, the concern that Andor's withdrawal might be a dangerous blow is addressed, but what we're getting out of it is the reaffirmation of characters' commitment to overcoming that blow. Which is just like how
Losing the Peace had member worlds officially reaffirming the Articles of Federation after the initial post-
Destiny squabbles.
So I do agree that losing Andor is a blow, but ultimately the goal is not to let it concern you in the long run and focus on what you have left - and that goes for Bacco and co in-story as well as for us readers.
