One of the bigger mistakes Salesforce made with Lightning is moving from purely transactional model to default-cached-no-way-to-purge model. Without letting a single developer to know that they did it, what are the pitfalls or how to disable it (you can't).

WRT Lightning motivation, sounds like a much better option would've been supplement older server-rendered pages with some JS, update the stylesheets and make server language more useable. In fact server language is still there, still heavily used and still lacking expressiveness so badly that it's 10x slower to prototype on it rather than client side JS...