Yeah, LineageOS can definitely help a lot. I have a Redmi Note 4X from 2017 with 3 GB RAM and a Snapdragon 625. It was fine running an older version of MIUI despite being a budget phone, but after switching to LineageOS it runs even better. But to be honest you don’t even need a lighter ROM like LineageOS if the phone was a good one at release. I also have a Galaxy Note 9 from 2018 which is running stock and that still feels great despite how heavy OneUI is. Often these older devices just need a reset to clear out all the junk that accumulates over years of use.
I think the questions over whether some newer phones can handle five or even seven Android version upgrades are valid, since that has never been attempted before (though I still like to see those commitments). But that is very different to saying every phone until now has magically turned terrible after 4 years, when it’s likely only running a version of Android that is, at most, two above what it started with.