An elite talent that will be 31 years old at the end of his contract. And he hasn't even played on turf, yet.
You're thinking too short-term. Resigning Semien and trading for a decent pitcher will likely add $30 million per year to the payroll, with something like $100 million+ on an aging player. And that's assuming we won't have to overpay for him like the Jays always have to do. You've now committed a quarter of your entire team's payroll to just two players, both of which will likely not be worth the money in just a few years. And you haven't even paid Guerrero and Hernandez yet, which are a couple 20 something players who will be looking for $10 million+ bumps in their pay, nor have you paid Ray or Matz, two players that contribute to the Jays' biggest weakness: pitching.
OR
You let Semien walk and take the compensation pick, then get your pitcher. Use the money you would have blown on Semien to lock-up Guerrero since he's going to ask for roughly that much anyway. Biggio keeps on getting on base and hitting home runs making him a perfectly decent replacement for Semien (and, hey, maybe he gets better at 2B. If Guerrero can, Biggio certainly can). Throw another uber talented but super cheap infielder onto the team (Lopez is looking good), and use the money to sign a good pitcher, something we actually need, when Ray and Matz walk (or resign them, if possible). With this set up, you've now addressed all of the Jays pressing needs without trading any prospects or throwing out another $100million+ contract on an aging offensive player that you don't need one bit.