I'm not certain of American Express' criteria for Centurion invites, as they have never been published. some people like to guess. I just asked for it. I'm certain it's no reward, though as current invitees have to fork out $10k for the first year.
It's interesting that you have done calculations of ROI, although as long as any card returns more than it costs, they are all good, since you could not have 20 Citi Select Visa accounts to multiply your return, the more cards the better. In fact, I believe Amex allows an individual to have only so many accounts. I have 2 charge cards and 2 credit cards with Amex, plus 2 bank issued cards. The changes to Amex earn, last year, actually made it easier. for me, as there is no need to worry which card gets which spend, as long as they all reach the bonus point limit.
I doubt any card suite could beat the return we receive from Amex, especially as we are in a situation, where, if we didn't want to travel, we could exchange 4 million points a year for about $40000 in cash. That works out to an ROI of 615% on points, alone, although I'd much rather use those points for travelling. The only 'investment' part of the card products, for me, is the points bank which grows, if some remain unused, so I will still be able to travel, once my spend reduces - assuming that frequent flyer programs are still around in 10 years. If I consider the interest earned on the average 300k owed to Amex, at any time, which sits in my accounts until they require payment, my ROI increases to 770%
I don't believe that Amex has devalued Centurion. Its costs have increased, dramatically, as have those of the banks. These financial institutions have to buy points from airlines, and as their value increases (in theory, 7-10% a year) so does the costs of providing the points. Many banks halved the value of points a few years back. Then Qantas made them all choose between Qantas and everyone else. Only MR Ascent Premium allows all.
I still agree that bank issued products are great for most people. Cent is used by 1/30th of 1 percent of the population so they obviously don't feel the need to please the other 99.97%.
We have to pay more superannuation, more towards Medicare, more for insurance, petrol, food, entertainment or get less for the same money. A packet of smith's chips used to be 250g. Now it's 175g. A block of Cadbury dairy Milk used to be 250g. Now it's 200g, or less. Tim Tams used to have 13 in a pack. Now it's 10 or 11. Cherry Ripe used to be 56-57g. Now they're 52g. Everything is worse value, due to inflation. Our card products are no different.
Enough rambling, for now