There’s a big gulf between those two, but for what it’s worth:
The motives behind trading these guys differ, of course. Trading Porcello gets you talent for an MLB-regular starting pitcher, with the Tigers likely believing that Porcello’s production can be matched or exceeded my making Drew Smyly a regular. With Scherzer you have the “plug in Smyly” option there of course, but it’s way more about getting peak value and return in a trade of a starter while avoiding having to think about an extension.
There’s no such thing as having too much pitching. But when you have a lot of starting pitching like the Tigers do, you at least have options.