^ Probably part of the agreement with Honda for taking over the PU assets, engineers and IP for the next 3 years. Whether it's contractually binding is another matter, but given the massive pace and result difference between Tsnoda and Gasly I can't imagine any other reason why he'd be kept on.
I don't know where Russell will come in against Hamilton. It's a completely new car so assuming they both get roughly equal testing mileage Hamilton shouldn't have any real advantage knowing the little nuances of past Mercedes.
Russell is hardly a rookie now either. He will be in his fourth season in F1. At that stage in Hamilton's career he'd won 11 races, fought for three drivers championships, winning one and would be challenging for a third time in his fourth season.
Hamilton was very much an exception in being given the keys to the best car for his rookie season and with complete support from 1/3 of the way into it. But Russell knows what it takes to win races - he dominated a very good field in F2 (comfortably beating the likes of Norris, Albon and de Vries), and should have won in Sakhir bar Mercedes bolting on Bottas's tyres, then a puncture on his own set.
Hamilton is a great, but isn't unbeatable. Alonso tied on points with him despite Dennis hating him since Monaco, Button beat him over their three seasons together and Rosberg managed to beat him in 2016, though the mental effort needed was enough to finish off his F1 career.
Hamilton is now such a well-rounded driver that I doubt many will expect Russell to beat him in the championship, but providing the car is good enough then Russell ought to win two or three races - whether he can extend that to a championship battle in 2022 or 2023 only time will tell. There aren't many drivers in F1's modern history capable of relentless determination and consistency over a race distance, time after time. Schumacher had it, Alonso still looks to have it (though not as quick as his prime e.g. 2012), Hamilton has learnt it and Verstappen is showing signs too. That's what separated Hamilton from Rosberg and Bottas and it's something Russell will need to learn if he's to beat Hamilton in however long they've got together as teammates.
Jack Russell needs some time to mature in a high pressure environment. I think it's foolish to expect him to jump in the Merc and be sticking it P1/P2 consistently without errors.
Horrible Johnny Herbert flashback to repeatedly calling him Jack Russell during F2 commentary. It went on for several races.