That isn't true though.
Take a look at Chelsea, they are on their way to have a dozen of GK and they keep scouting and getting more, loan some and keep others with no playing time. 4 GK in their squad and 7 on loans beside those who were bought for next year.
Joan Garcia gave his ok to Arsenal move when there was zero chance he will displace Raya.
It has proven time and time again that most players follow their agents, and agents follow the money above anything.
Suzuki was one of the few who didn't, respectful tbh as he would have had more spotlight and money in Manu than in Japan or Parma.
I don't think the plan is either option atm, we aren't phasing out MATS or committed to him.
We are aiming for bringing a competition and letting them sort it out later, sort like Bravo & MATS back in the day.
Whether it is a high potential GK (Backhaus, Garcia or Verbruggen) a cheaper veteran (Valles, Bulka) is to be seen.
I think long term a combination of both would be the best. Especially if we would take a punt on someone like Backhaus who could become very good but is still a long way off (but if he's cheap then he's worth a risk as much as anyone else) then we would clearly need a veteran who can perform immediately and compete with MATS/Szcz while Backhaus would develop in another club.
Alternatively, if we sign a younger GK to start then we will need a veteran who is willing to sit on the bench.
Not necessarily with the players you've mentioned but I agree we should repeat the scenario we did a decade ago when we eventually move on from MATS. Hopefully we'll have his replacement lined up and ready before he leaves but with how this club has worked for years that's unlikely.
If we spend big money on Garcia or Verbruggen (he's probably too expensive anyway as is Diogo Costa) then they are clearly coming to be starters within 12 months. De Zerbi rotated Verbruggen with Steele last season when he was adapting to Brighton so he would probably accept a bench for a season here too but then would want to get his fair chance. So if that's the way we'll go then we have to move on from MATS by 2026 as said.
Chelsea are a special case, no idea how they convince all these players to sign with them. Can't be just the money or agents.
Barca never worked like that though but to some extent we maybe should once finances are better. We don't use loans often or good enough. They are usually seen as "giving up on players" rather than helping their development. Hopefully Fermin's success will change some people opinions. And from club's perspective is important to loan players to the right club and at the right time. Timing was usually our problem in the past, we just kept talents to stagnate in Barca B for too long.
And we rarely sign a player and leave him to develop at his current club or loan him elsewhere. Pedri is the only example I can remember and it worked great both for him and the club. We surely also missed out on many players because we wanted them to start in BA rather than loan them to continue developing on a higher level.
Not saying we should go crazy like Chelsea trying to sign all wonderkids but when you know you have an aging starter (not talking exclusively about GKs here) that is still good enough for now but will need to be replaced in two years then why not sign a player and let him get experience in Betis/Las Palmas (or potentially even a better club if he's already good enough) for a season or two before you bring him to the club? Other clubs are doing this successfully, why couldn't we?
For longterm GK that's probably how we should do it. Like Chelsea (again) for years were loaning out Courtouis to Atleti until he was ready. Identify and sign a GK and then let him develop on loan.
Not exactly the same case but Liverpool did a great job with Mamardashvili. AFAIK it's a future transfer not a loan because he prefered to stay at Valencia rather than going on loan to Southampton (IIRC). But they guaranteed their future GK while they still have Alisson.
I agree some young GKs will accept warming the bench to sign for a top club. Ofc you never know when a starter gets injured and you will get a chance but as you've mentioned the Joan Garcia example in his place I would never go to Arsenal to sit behind Raya.
GK is not like other positions, there are rarely rotations and if you are behind an established goalie in his prime years then it's very unlikely you will play unless you are "lucky" he gets seriously injured. And tbf with the amount of long term injuries increasing, also for GKs that's probably something that players/agents count on too.