The answer to this one largely depends on how you feel about Druids, doesn't it? Modern day druidism is an accidental hybrid creation of Victorian scholarship and the Romantic movement, idealising nature and projecting backwards onto our ancestors. More to do with revulsion against soot- choked cities and the artificialities of etiquette than anything involving, say, primary sources.
Of whom, Caesar. What he had to say about them is certainly not generous- he was justifying ethnocide- but it is information. He felt they were a political threat, the representatives of a different kind of social order which could never sit easily alongside Rome.
The religious argument- remember Caesar was also Pontifex Maximus, and considering how sincerely he took that (hardly at all), we can infer that there wasn't much theology behind his decision. What there may have been was envy.
He describes the druidic faith as what I would call not a million miles off the Aztec, except slightly more sustainable. The priests still wielded the fundamental threat of a hostile world- note well, classic druidism is based on the harshness and cruelty of Nature- that needed service and bloody sacrifice to appease; but the European end of the idea was less millenarian about it, maintaining enough pressure to bend society to their will, but not enough to cause society to begin to come apart.
He was seeing a religion as a tool for social control, that was highly effective but also completely incompatible with his own. (If Caesar wasn't an outright atheist, he was certainly deeply cynical about it all. Cynical enough to keep a straight face when confronted with true believers, and play them accordingly.) What sort of deal could you come up with between these two impossibly incompatible religious- political systems that doesn't involve the destruction of one or the other?
The answer that you get, the deal, will contain the seeds of what is going to happen next.