The problem with smackdown type characters is that cheesy multiclassing tends to be the order of the day. Why be a fighter 20 when you can be a fighter 4/PsyWar 2/Bbn 2/Exotic Weapon Master 2/Warmind 10? The majority of the advice you'll find on said boards is against straight-class characters. So the fact that Ftr 20 doesn't show up on the smackdown boards other than builds like SnowSavant's gattling tripper doesn't mean that fighter 20 is a non-viable class; it means that it's generally a non-optimized class. Those are different things.
The unique fact about fighter is that it multiclasses extremely well from levels 2-12. Sorceror is usually a class people multiclass out of as soon as possible. Wizard builds sometimes take a few more levels of wizard to get a bonus feat but most wizard builds ditch the wizard class pretty quickly too. The same is true of cleric. Druid, monk, and paladin sometimes show up straight-classed at 20th level but that's more because they have abilities that only show up at high levels and can't be duplicated by prestige classes than because they are viable straight-classed and other core classes aren't. If you want a character that has Quivering Palm, Diamond Soul, Abundant Step, lots of Stunning fists per day and very high unarmed damage, you have to be a single-classed monk. Once you get spell resistance as a monk, it's only worth having if you continue to play a single-classed monk character because otherwise it falls behind the usefulness curve. Wildshape has some stopping points but if you want to get the most out of wildshape (without sacrificing spellcasting), you need to play a druid. Paladins have prestige classes that allow them to advance their mount, their smites, and their lay on hands, but no prestige class that allows all of those abilities to advance. So, if you want to maximize mounted smiting, you've no better choice than to play a Pal 20. All of the classes that show up on the character optimization boards straight-classed show up that way, not because of any inherent power in the class, but rather because they have abilities that require being straight-classed to maximize. The fighter doesn't have any such abilities. Every single abilitiy the fighter gets works just as well for a multiclass fighter as it does for a straight-classed fighter. So, the only thing that can make a straight-classed fighter optimal is feat synergy that is impossible to get without the sheer number of feats that fighters have available to them. (Feat synergy is possible either through feat chains--Dodge+Mobility+Spring Attack--or abilities that work well together--for instance, Jormundsbrod (or however it's spelled)+Power Attack+Cleave+Dodge+Mobility+Spring Attack+Elusive Target+Combat Expertise+Improved Trip, all of which work together quite nicely against multiple opponents).
The question of whether a class is viable is distinct from the question of whether taking it is optimal. Even though I don't know of any Ftr 20 build that is truly optimal, I'm pretty sure I could take a properly equipped fighter and sit down at any table in an RPGA game (or any other game that doesn't both allow and expect builds with Teflammar Shadowlord, Cheaters of Mystra, etc) and not be embarrassed by my inability to contribute in combat in a level-appropriate manner. I might have a boring time if the entire game were delicate negotiations (so no intimidation) and investigation, but I wouldn't be a slouch in terms of combat-power. I've played D&D long enough to see just as many characters inferior to a single classed fighter with NPC feats as are superior to said fighter. A lot of them are even fun and playable in most games.