I'd like to start off with a counterargument that unless the opponent survives to a high enough damage, or the battle is genuinely one-sided, they will not charge over the course of a single stock that often. We've seen the details of Final Smash meters and how they work, from their slow charge rate when taking damage (it's approximately 200% without any other factors), their even slower charge rate when attacking somebody, and the fact that you lose charge if KO'd with full meter on. We know how the meters essentially work now, all we would need to do is create a proper theoretical counterplay to the usage of Final Smashes and the meters themselves.
Sorry, but this is provably incorrect. In many, many videos featuring FSM, a full charge normally occurs at around 120%-140% for the "losing" player, with the other player's FSM filling soon after. Also, 99% sure that it isn't a blanket "lose half on death" as per several different videos. I'll be happy to confirm when I'm at home.
As for Counterplay, I look forward to hearing of the counterplay available to the FS of Daisy, Peach, and Diddy. That's far from the problematic list, but that is a strong start.
Now, as for my justification of the inclusion of FSM, I would like to start by pointing out that I'm not desiring them to be in combination with our current competitive Singles. I'm only fighting for its inclusion as a third main event rather than an addition to an already pre-existing event. But I digress.
With this in mind, I think that FSM Singles would be appealing to a far broader audience than regular Singles would ever hope to achieve. Final Smashes may have this hefty amount of power to them, but as we have well seen, audiences and spectators absolutely love Final Smashes being used (more often than not, you hear them cheering it instead of any other reaction). It's not often that I've seen a Smash tournament usually have this level of excitement unless some sort of combo string happens, or a major upset occurs. I think it would just grab the attention of a broader range of people due to how exciting they are, thus resulting in a potential playerbase growth and viewership growth.
"Viewers love it" is the worst justification you can use. It's one thing for a viewer to like something and an entirely different thing for competitors to have to deal with it. That should be the primary base for inclusion, not the viewers. The quality of gameplay should not be the cost of inclusion and, if the gameplay suffers, so will viewership once the novelty dies. (See Bayo and Evo 2018 top 8)
Now regarding the fact that I would want them to be included as a third main event, I mean it for a rather interesting reason. If run as a third main event, there would be no need of talks of the competitive viability of FSM, since I believe the community wouldn't mind it staying as its own event, just so long as they can run Singles and Doubles without FSM in the meantime. While it would create a scenario where tournaments could run for a little longer than before, I think that this addition, rather than a change, could be well-accepted by the community rather than shunned. The community wouldn't need to bother arguing on its inclusion, so it won't hurt anybody to just make it its own thing.
Why would competitive viability be out just because it's side event? Are doubles not competitive because it's a side event? And what side event do you remove to include it? Squad Strike is something that would easily be the primary side event because, unlike FSM, it's not degrative to gameplay and it's looked at positively. How about Squads? Ironmans? Pros vs Scrubs? OTHER GAMES? You only have so many setups and so much room. Removing another side event just to have it would hurt people who don't want FS jank, plain and simple.
(Addendum: Since we have something known as "Custom Balance", if Final Smashes prove to be too powerful, we could simply tone down the damage of the characters (and subsequently, Final Smashes) to ensure that Final Smashes don't come too often and don't kill as early, if they were to be their own main event.)
If you potentially need to REBALANCE THE GAME just to make it work, that should be a MASSIVE red flag for you.
Wouldn't you agree that just simply making it a third main event would justify its "inclusion", alone? Or should I elaborate on some of my statements?
Clearly I do not agree. Singles > Doubles > Region vs Region Matches > 3v3 > Ironmans > >> >> >> FSM enabled side event.