Hi @bkm,
I do agree that a LTS would be great, but we also have to keep in mind that maintaining several versions at the same time requires a lot of resources that no-one in this community seem to have/be willing to invest until now (at least I have heard of it but seen no one drop a few $1000 on the table specifically for it).
Anyway, please forgive me to correct you on one important point:
This is not true, since Frappe/ERPNext uses GIT and semantic versioning allowing therefore anyone to choose their version of ERPNext.
A quick search on the forum will guide you through the different steps of this process.
For example:
Of course it will not be as easy as using only the easy install script (there are only two additional steps…), but I do think that if anyone has customizations that can not be easily ported to a new version, they would have the internal knowledge to checkout a specific version of ERPNext and Frappe before migrating a backup to a new server.
And there is always the possibility to fork a specific version if you don’t want to update. Forking will allow you to easily port some important commits merged in a future version (security issues/major bugs/…). In this case you would be maintaining a sort of LTS.