It's quite intriguing. At the moment Tuchel hasn't agreed to join and I can quite see why. Avramovich has sacked managers in mid-season on five occasions, including Frank. On two of those occasions he brought in Hiddink and in the others he brought in Benitez and di Matteo. Hiddink and Benitez went at the end of the season in which they were brought in. Only di Matteo survived into the season following his appointment. Despite finishing sixth in the PL he won the CL to earn a permanent contract, but even so he lasted only until the March of 2013. The longest lasting managers at Abramovich's Chelsea have lasted more than two years but never more than three!
So it will be interesting to see if Tuchel goes whether he goes on a contract initially only to the end of the season - in which case he may well face the sack if he doesn't finish in the top four - or whether he gets a "permanent" contract. Personally I'm not sure it makes much difference. Abramovich's "hire then fire" style appears to be a style much admired in his homeland where success reflects on the owner rather than his underlings so a "permanent" contract may be little more than reassurance to Tuchel's bank balance rather than a guarantee of survival beyond the end of the season.