From a sovereignty perspective, does a launch from continental Europe actually matter when Arianespace/CNES/ESA has been launching from France for decades?
Doesn't it matter if this part of France is an ocean away?
There is no sovereignty perspective because Europe isn't even sovereign to begin with. For starters it doesn't even have a common army
But French Guiana is a geographically distant ex-colony, so there will always be a higher probability of future inaccessibility. Same applies to New Caledonia, Martinique, etc.