It should be clear, and apparent to anyone paying attention, Stamkos and Tavares are significantly different at this stage.
Stamkos is a severely declining player, who was fed by Kucherov, and is nearly a -40 the last two seasons. He is a disaster 5v5 and is mostly a PP merchant at this point. He is no longer useful as a C, and can barely skate.
Tavares has declined some, and his pay should reflect that. But he's good as a C, and is a positive 5v5.
Comparing these two is a bit extreme, as they aren't comparable. Stamkos signed for 5 years x $8 mil. Tavares won't sign for that term, or dollars. Meanwhile, Tavares is probably worth multiples of Stamkos at this point.. I mean, Stamkos is really worth... maybe $2 mil in a sheltered role... at best.