The purpose of a supercarrier is sea control and from that can come power projection. During the Cold War the USN planned to take the fight to Soviet sea denail forces at sea and then advance to destroy their bases, only then would it start to support Marine landings etc. In the Falklands the 2 RN carriers would have cleaned-up any Arg naval forces that went to sea, abandoning other tasks to do it.
So any country which desires to control the sea, even if locally and temporarily, needs powerful fleet carriers to clean-up local enemy naval power.
The costing for a CBG seem to be based on clean sheets of paper, which navies who'd want a big carrier force never are. Even the RN will have aircraft available for it's big carriers, bought and paid for elsewhere. It's all part of an ongoing naval programme, to divorce big carriers from the rest of the navy is false.
Not really. The need to become a force projection, deep water player, requires a completely different sort of escort vessel, and far more of them, than would pre-exist. A good example of this is the PLAN, they would be starting from a clean sheet of paper.
A less obvious, but equally valid, example is the Royal Navy. The RN has procured several new SSN (Asture class) hulls. These are sufficient, presumably, for current RN needs but would not become part of any future CBG. Even current DDG & FGG force is insufficient for needs. The new Type 45 destroyer is an eight ship class, few enough for current UK needs without carrier escort duties. A single carrier group would require five, perhaps six, of these platforms, along with two SSN, a replenishment ship, and a separate underway support group in the case of extened deployments (say a fleet oiler and two DDG as escorts). This single CBG has absorbed the entire modern RN, save two SSN. Since the UK is planning two decks, you now need to buy an additional five-six Type 45s at $1.7B a pop, just to escort the second deck. The UK will also need to build at least four more Astures for non-carrier escort work, at least six more modern DDG to provide escort for Amphibious vessels, independent operations & contribution to NATO sanding forces and the EU military structure.
Much the same can be said for France, whose escorts are horribly long in the tooth, save the Horizon class FFG and proposed FREMM ships. Both of these classes are, unfortunately, too slow to really operate with a battle group (Horizon tops out at 29 knots, FREMM is supposed to be 27 knots), rather like the OHP class FFG that the U.S. was forced to use in the carrier escort role while the Spruances came aboard. If the French want to run a real force (at least two, probably three decks) they will need 10 more escorts MORE capable than the FFG's currently planned.
As noted earlier, you can go on the cheap, but if you do, expect someone to start punching holes in your carriers (something that really upsets the taxpayers).