Somebody wants to learn a language properly and you're saying "don't bother". I don't get it.
To put it another way, it's a bit like trying to to learn about how to write in the style of a judicial opinion when you're still learning how to talk to someone at the bar.
Yeah. I can see that. Let me see if I can expand.
1. Keigo falls into the category of “specific purpose” (SP) language. SP language can be relatively simple things like tourism language, or it can be very deep and technical like scientific, medical, engineering, etc. SP language is typically taught and learned on a need to know basis, and it’s totally possible to be highly fluent (usually measured in terms of general proficiency) and not know anything about SP language.
2. Keigo (and this is way more than basic formal and honorific language) is used in very specific situations. As a foreigner, in almost all cases that matter, there will be an interpreter or the foreigner will be trained in keigo.
3. For reference, I think that most of the keigo that I would recommend a non-native speaker learn can be found at this link (it’s a small and easy to learn list):
https://www.fluentu.com/blog/japanese/japanese-keigo/
Furthermore, I think that most of this language is more useful receptively (listening and reading) and will almost never need to be produced (spoken or written).
4. As far as proficiency goes, most keigo would be ILR 5 / CEFR C2, with the basics linked above being ILR 4 / CEFR C1 (used dynamically… much lower as memorized set phrases). When you’re at that level and needing keigo, you know you are lacking in keigo and make an effort to learn it. Before that would probably be a learning sequencing error (other than nerdy curiosity).
Beyond learning the content in the link above, most foreigners will not find themselves in the context of needing to know or use keigo. As special purpose language, it should really only be pursued if necessary. Otherwise, the knowledge learned will most likely be forgotten due to lack of use, or it will be used awkwardly and possibly inappropriately (as I mentioned before).
If it’s just a nerd itch to scratch, then go for it. For most people, I think there are more compelling aspects of Japanese to spend time on.
If you put it like that - OK. I was under the impression that you were implying "learn no Keigo at all beyond introductions (except Teineigo)", which would seem rather odd to me, as I've definitely heard 行ってまいります, Xでございます, etc. in Anime and movies.
If you're talking about the advanced uses that even many native speakers struggle with (unless properly trained), then yeah, maybe it's specialised vocab that you won't necessarily need.
To me, this is the part that is keigo. The other stuff is relatively simple and early in terms of language acquisition.
Assuming that others share my definition of keigo may be assuming to much on my part.
In learners' resources (where Teineigo is introduced usually right at the start), my impression is that Teineigo is not included, and that Keigo starts with Sonkeigo and Kenjougo. That's still introduced comparatively early, but much later than Teineigo (I think, it's in Genki 2, for example).