I don't believe a keynote speech has to be delivered by the founder of the language or framework. The founder can still deliver their speech elsewhere in the schedule.
When the Crystal programming language Conference was held in 2021, the organisers asked the Ruby creator ("Matz") to give the keynote speech. I thought that was a lovely gesture from the conference organisers.
Having a different keynote speech (not from the founder) might give the audience an unusual, suprising and even inspirational presentation.
Two made-up examples:
1. Keynote speech: Ruby on Rails: From zero to hero?
Lynsey, a cafe worker, lost her job during the pandemic. During lockdown, she began to learn RoR with no idea where it might lead her. She had never programmed before and found the process complex and maddeningly frustrating - punctured with occasional moments of joy. What did Lynsey learn as a beginner to RoR? And what happened next for Lynsey?
2. Keynote speech from our founder [insert name]
A look back to our successes and progress over the past year. And a look forward to the future.
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I know which one I would rather listen to.