Kind of like a Pyramid scheme. Need to sign up more new guys below me so I can move up and get one of those coveted spots above me. :D

In the US the FAA requires currency to be maintained on each variant. That would be a nightmare and extremely expensive for the company. Different rules in Europe.

Maybe having 1 or 2 crew domiciles is easier to keep current than having multiple bases like Delta.

So you're going to solve the "issue" of scheduling flexibility/trip mix by doing something that would require even fewer pilots in a category?

that is not what I meant, I meant smaller countries like in Europe is easier because they have 1 or 2 domiciles, it will not work in USA unless you want a QOL with a legal 9-15 days 'one' trip a month like used to be in the 1970's, i.e NYC crew can go to Europe, then west coast to Asia/Sydney back to either west or east coast. with all legalities adhered as you 4 models to choose from 332, 333, 339 and 359.

ps I saw on APC that A321 pay is different tgan A319/320, does that mean A321 have different group of pilots eventhough is heavier as some suggests.

I am sorry too much, just a guy with no sense to make some 2cents

319/320/321 same type rating so pilots fly all three variants, but 321 carries more people, so it pays at a higher hourly rate.

Same for the 737-900 which pays more then the 800 and 700 (when we had 700’s)

Thanks for clarification

They fly from Europe to the US with two man crews for anything under 10 hours. That alone probably solves most currency issues, however as I mentioned in the US you need currency on each type. I don’t think that is the case in Europe. Many US pilot contracts reward a loss of currency leading pilots to in some cases prefer to go non current. That is generally not the case at Foreign airlines where a loss of currency has pay issues.
As far as how bases are structured that again becomes a contract issue. If you have a poor contract with few or no pay rigs and minimal works rules the company can structure pilot basing almost anyway they want. If you have a solid contract with good rigs and work rules the company has to be far more careful in where they base pilots to keep credit low.

Pay Banding Summary
 

350 - If you want to be the king, climb the hill and knock me off
330/765 - too cultured for ULH destinations, should be pay banded to the 350, since they are within 15 pax anyway.
7ER - The ER trips should be 330 pay, We don't acknowledge the existence of the 321 because it can't do a vertical afterburner takeoff like the F-757.
737 - this plane is horrible, we deserve every penny of additional pay. Wait, wut,? half the planes are already in the same band? (321 /-900) When can I AE out of this antique?
320 - The $$ would be nice for a couple years until the 319/320 retires, but banding will crush my seniority once all those 737 pilots dogpile on top of the list.
220 - We have airline jobs. Who cares about banding, every. single. bid. is. a. raise..

777/M88 - RIP, may your pilots find peace and happiness flying in French...

717 - Not even worthy of the list