@ljharb brought this up in TC39 the other day...
Ponyfills should absolutely be encouraged, I think there's plenty of benefits to using them. However, there's a valid use case of polyfills that gets ignored:
When you have nested dependencies that have less backwards support than you do they won't use ponyfills/polyfills. Now you can't use that particular dependency without using a full polyfill unless you fork the dependency (which could also be several deps deep).
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