> Why all the mediocre product that's the tail end of pumping billions of dollars into R&D staffed by largely the same folks Tesla has been hiring?

In an established firm, existing product lines pay the bills. No company is an island onto itself. Companies have partners, large clients, relationships, etc.

Bottom line? To keep existing product lines healthy, you must pay respect to your loyalties. You start seeing the behavior of purposefully undermining the new product to keep the old alive and kicking. You start using sub-standard parts, just to keep purchasing from the same suppliers. You keep on old hands that actively despise the new way of doing things. And that's just the tip of the iceberg.

Frequently, starting with a clean slate is a huge competitive advantage. No baggage.