For the very reasons you have given I think there is a longer term plan in place(longer is a relative term). Sam obviously does not want to be a stop gap manager anymore.
He was offered the opportunity to manage until the end of the season before anybody and turned us down.
As you correctly say we certainly did go after Silva and Fonseca and Tuchel have also been mentioned although the level of contact with the last two is unknown.
So, what happened to make Sam change his mind... in spite of what people say he isn't motivated by money anymore. He walked out of the Palace job and away from an existing contract.
Are people so silly as to believe that we offer him an 18 month contract but will sack him after five .... and Sam didn't see that possibility coming and that scenario was not the first item discussed between Sam and the club.
Everton is the biggest club Sam has managed and now will ever manage. I believe with Sam it is all now about legacy.
It is obvious he believes he is a better manager than the clubs that were given to him previously and I can imagine the England fiasco must still really hurt. I think he wants to redeem his reputation and at the same time prove his quality as a manager... to everybody else.
I believe that he will be here next season as manager at least . After that I don't know really. There may be the offer of a different role in the club or instead the offer of eighteen months as manager with financial backing and it is up to him then to convince everybody he should stay longer.
Either way, if results are reasonable from now until the end of the season I see him here next season.