It isn't quite as simple as that though. Removing positionals is a given, it is the re-allocation of actions that need to be considered.
Like it or not, using the current tank defensive framework, you need a short mit, 90s mit, 120s mit, invuln and a raidwide mit. at the very least. So what, from Dragoon's pool of attacks, can be changed? Battle Litany can easily become a raidwide mit, Life Surge and Lance Charge could be changed into, say, short and 120 second mits but that still leaves a 90s mit and invuln you need to fit in.
There is also a consideration in how busy a job can be in a certain time frame. If there is no slots to use for oGCD defensives, it will just become painful to play around, this is the whole reason GNB and DRK gap closers were changed after all.
There is also no point in giving a basic kit and saying, 'that's good enough' when it isn't. Going by your metrics, add a stance, adjust self defence skill, invuln and a personal/raidwide buff, that is less defensives that the tanks currently have. If you want the jobs to be taken into serious content, it isn't enough, so the content is either going to be balanced around the lowest point, or they balance it around the current tanks, leaving this half finished tank behind because it offers nothing.
There is more work that goes into this than I think people realise.
I once tanked Delubrum Reginae as a Monk as there were no other tanks among the 23 other people. Despite having a tanking potion and a lost action for defence (the other was for enmity generation), I still felt I was missing out on my defensive capabilities. Remember, Monk had Riddle of Earth as well, so I did have extra. But there were times I still felt too squishy. I also think there were times where I needed a defensive for a tank buster, but had no choice but to take it to the face due to defensives being on cooldown, but I do not remember exactly.
The point being, whilst it is possible to fashion yourself into a pseudo role, you still weren't quite as good and it didn't feel quite right, something was missing. This is why the system Op is proposing cannot just be a half baked idea, it needs to have more careful considerations and be done properly for it to even have a shot at being successful.
With Dancer, you still need a way to mass heal people quickly, especially in single target damage, Curing Waltz isn't going to be enough and, assuming the dances are still going to be a DPS gain, they will be used for that rather than the burst healing they provide. As for RDM, you will need similar things, a Vermedica is obvious, but a better single target heal, maybe some oGCD heals etc. But this then comes onto another crucial point, if they need to trim down what is already there to fit in the new actions, would that be ok? DNC not having their Fan Dances anymore or RDM not having a melee combo as examples. These buttons for their new role have to come from somewhere and whilst some jobs have some space, not every job does. Something will likely need to be cut to make room, especially in changing a job into a healer.