I don't disagree but GrapheneOS for instance only supports handsets for as long as Google does. I think some of the other distributions also follow this rule.
I don't think it's fair to say security only decreases if you install Lineage. Yes, you'll lose some security during the boot process and with debug mode, but you also get newer security patches fixing exploits your older OS would be susceptible to.
Because it's not a project with a goal of breathing old life into provably less-secure hardware.
If you want to reduce e-waste, know you will be reducing your security, and use Lineage or whatever targets the most devices,
not the most security.