I think it's also about:

- later recycling frequencies

- reducing maintenance burden (by not maintaining 3G)

- as far as I remember 3G as serious security vulnerabilities

Also most smart phones only a lifetime of 3-5 years at most, and even then people tend to replace them early due to braking them and for vanity.

So there are probably not too many smart phones around which do not support 4G and which are not a security liability. Through probably a bunch of fallback feature phones or other "less common" (then smartphones) kinds of phones.

Also it differs by country.