do you think speculation is necessary for ethereum adoption?

Speculation seems inevitable, and I think it does contribute to adoption. That's different than necessary.

I can imagine a world where it doesn't happen and people still use Ethereum. E.g. imagine a culture where speculation/gambling is so socially ostracized that people just don't do it.

agree. the likelihood of such a world emerging almost feels like a Hobbes vs Rousseau style debate so not obvious to me either way 

The dark side of implementing a culture of ostracization is alienation. Ie middle America’s reaction v. coastal elites