Don’t forget they wiped out the cultural heritage of the Gauls and Brits.
These the same Gauls whose culture and language remained vibrant and present in archeological finds until the arrival of the migrating Germanic tribes?
The same Brits whose culture, likewise, survives until the Anglo-Saxon migrations pushed them into enclaves in Wales and Cornwall?
Wiped out most of the Gauls outright, really. Estimates of Caesar’s conquest was 1 in 4 Gauls dead, 1 in 3 enslaved.
No, accepting Caesar's numbers ("Caesar considered that a million Gauls had been killed, and a million enslaved.") with modern estimates of the Gallic population (~6 million) results in 1/3 dead or enslaved. The issue is that mixing-and-matching numbers like that is highly questionable, even if you want to take Caesar's conveniently round estimate as literal and accurate.