Biggles, there are decals and then there are dry transfers.
There is simply no comparison, as even the finest decals have transfer film that can silver if conditions aren't just right. Yes, you can apply a coat of Future, but that isn't foolproof.
Dry transfers (or even when applied with Archer's fantastic Wet Medium Paper) are an order of magnitude above any waterslide decal because there is no chance of silvering at all. They admittedly AREN'T cheap, but quality comes at a price.
And since the quality of kit-supplied decals varies enormously, dry transfers are there for the modeler who is willing to go the extra mile for a better solution. They're not for everybody, and not for every budget.