Old super star players have a high ego, no one will admit if their performance are declining due to get older and keep demanding high salary. There's no league willing to pay high salary for a popular player without good performance except Saudi Professional League.
So their options are either retire or joining Saudi Arabia, this can't be denied.
That is much more realistic than hoping to get a big salary in Europe and the impact of declining performance due to being no longer young is not the right choice because any club will definitely take many things into consideration. Sergio Ramos has had big offers from Saudi Pro Legue and MLS clubs, but so far there have been no further talks regarding his future. Sergio Ramos can still play for the next two seasons and choosing to retire may not be a good idea, it's better to accept an offer from Sevilla because reportedly they have reached an oral agreement.
If you look at Fabrizio Romano tweet it seems that a true verbal agreement has been reached, which means it could be that Ramos has agreed on the amount of salary he will receive. However, on the other hand, I don't know for sure why Ramos is reluctant to join the Saudi Pro Legue. In my estimation, Al Ittihad does not want to give the amount of salary that Ramos wants, in the sense that he is not unable to.
Or another possibility is that Ramos will go to the SPL in the following season, after defending Sevilla for one season first. It is difficult to predict because Ramos has free transfer status, he can go to any club without transfer restrictions.
In some of the media that I have read there has been an oral agreement between the two, but the issue of discussing salaries has not been discussed further because, as Besiktas said before, they have difficulty meeting Sergio Ramos's too high salary. I also see Sevilla being far less likely to agree a huge salary for him unless as you say Sergio Ramos is willing to be given a salary below what he wants.
Speculation regarding the future of Sergio Ramos seems to be getting brighter and he will have a high chance of returning to his home club when he starts his career because he has reached an oral agreement. I think the transfer free factor is keeping Sergio Ramos from deciding so quickly about his future and maybe he is considering playing with his home club before retiring or going to Saudi Pro Legue.