Actually your poll was for just the last year. Since campaigns can last years, your poll skews things a bit
Very many? How do you get that from the poll?
5e campaigns dont really last "years".
Designers designed its experience point rate to normally reach level 20 in less than a year.
And. Most players dont ever play a campaign to level 20.
If by campaign, you mean an indefinite number of characters that are part of the same setting and the same saga, then sure, that might last years and decades.
Even so, what players are reporting to be true in their own gaming groups for almost the last year and half, is representative for how players are playing today in 2023 generally − especially in the aggregate.
Also I am not too worried about the implications deriving from the poll because they are easy to verify precisely. All one needs is full access to the DnDBeyond database. In this way, one can know precisely the frequency of nonmagical nonmulticlassing Fighter characters that update in ways showing ongoing use.
What percent of all characters in use are Fighters?
What percent of Fighters never multiclass?
What percent of Fighters do ability score improvements rather than an other feat?
What percent of any feat is nonmagical?
We can know the EXACT percentage of
nonmagical Fighters − from the DnDBEyond database.
I estimate that somewhere around 48.8% of all Fighters at level 8 are nonmagical single-class Fighters.
Thus roughly 6.5% of all characters are "old school" nonmagical Fighters.
In other words, there are about as many "traditional" Fighters in 5e as there are Monks in 5e.
Again, all of these estimates are simple to verify with access to the DnDBeyond database statistics.
If in 5e, the frequency of old school Fighter players is about the same as the number of Monk players, that would explain why many players have one in their own group, and why many players dont have one in their own group.
All of this is surprising because, anyone familiar with D&D 3e, and especially if familiar with D&D 1e, probably assumes without doubt that "everyone knows" a Fighter is a "nonmagical" class.
But it might be not "everyone knows" this.
4e already began speaking about "Martial magic", and in 5e gaming fashions are shifting with masses of brand new generations of players.
At the very least, there appears to be "very many" players who think of the Fighter as magical class for a magical game.