We (Svix, webhooks infra) actually help our customers directly write to Kafka topics, S3 buckets, SQS, etc. and have for a few years now. There are definitely people that adopt that, but receivers as well prefer the simplicity of webhooks.
Do you see receive-side customers as prepared to outright reject paying for vendors that only offer non-webhook event streams if there's a webhook-ful competitor available? Or is that preference more of the "well, it's easier to add a webhook route to our existing webapp than it is to run a stream consumer/cron/whatever, but neither of those two is cost- or effort-preventative" situation, where customers don't consider event delivery systems to be the main differentiating factor?
Though the data I do have: how many people adopt these advanced endpoints in practice (as we offer these), and it's less than webhooks.