>don't offer lifetime plans
Yep! https://successfulsoftware.net/2008/09/08/should-i-give-free...
I have been reading Andy's blog for some years now. Though the main focus is on his own products and his product development experiments and experiences, many of the posts have good advice for those wanting to create their own products for sale. There are also some interesting interviews with other product creators.
And for those aspiring to find the lifetime plans by startups, go look through offers on AppSumo.
To be fair though, the chances of your service still being around and GP still getting value from it 13y later were pretty slim weren't they? After all, you listed some other SaaS products shutdown in the interim in OP, did any of those have lifetime [of the product] plans?
Of course, I do realise non-recurring sales (coupled with the non-recurring sales not continuing, growth in users slowing) hurts those chances itself.
It's great to hear you got some use out of the tool and hopefully continue to do so. I look forward to your next life update in 13 years when Preceden makes the HackerNews homepage again :).
PS for any aspiring entrepreneurs: don't offer lifetime plans, and definitely not for $19, lol.