OP - Of your 5 criteria, 2 will change by the very nature of what it is to be acquired. The other 3 are essentially 1 (how excited are we about this change?)

Culture: there's different people around, now. So culture necessarily changes.

Flexibility: bigger companies are less flexible because there are simply more people. Even if every individual is more flexible, it doesn't matter.

Enthusiasm, spirit, and happiness - are these not synonyms, in this context? And if they are, are they not reactions to external factors (culture & flexibility)?

I would suggest your question may be tautological: big companies will always ruin small companies from the perspective of someone who likes small companies because the small company became big.