Hi [First Name],
Thanks for letting me know — genuinely. Most people just go quiet, and the courtesy is noted.
[Competitor] is a solid choice, and I hope the rollout goes smoothly. One question, if you're willing: what tipped the decision? It helps me more than any win would.
And an open offer with no expiry: implementations don't always go to plan. If in six months anything's not working the way it was pitched, I'm one email away — and I'll remember the context.
Good luck with it.
Best, [Your Name]