Why does Thunderbird keep upgrading?

I've been using Thunderbird 2.0.0.24 for years, and it works well. The
last time I looked, TB had versions that (from what I recall), were in
the teen numbers.

Since I regularly use both Win98 and XP, version 2.x is compatible with
both OS's, and I dont want to be downloading the same email more than
once.

My reason for posting this is to ask WHY Mozilla keeps upgrading TB?
After all, email is merely text and sometimes an attachment (usually
pictures). What can they possibly do to improve TB, especially that many
versions? Ver 2.x does everything I need.....

Is this just another example of Mozilla BLOAT? It seems they are
notorious for bloating Firefox, so do they feel a need to also bloat TB?