It's not character viability, it's perceived metagame stagnation versus variety of valid choice, and how those are reduced and increased respectively across the majority of the cast. Marth doesn't deserve viability any more than anyone else's, and fortunately, more than just Marth benefit from customs, and I've yet to see a character become un-viable in a customs environment, a detail you seem thoroughly entrenched against despite all arguments.
Both of my views there, I seem to recall you vocally disagreeing with outright. Fortunately patches help with both of these, but as someone whose geographic situation means I watch a lot more than I play, I get really tired of watching the same characters do the same things.
Customs directly mix that up for me, and consequently, I can (as mentioned quite recently) only really enjoy watching when there are people like DKWill or JohnNumbers running unusual characters, or when there are customs switching things up.
Either way, it's clear that our circumstances are entirely opposite, as are our stances. You enjoy your meta, I'll enjoy mine. I'm not forcing you to play mine, after all.