That's probably true. In fact, $100 million is probably fair regardless of the timing, as long as it ends in the same year. 6/$100 is probably fair this year, 5/$100 is probably fair after this season ($12 million for the final arb year and $22 million for 4 additional seasons after that). $100 million for 4 seasons would probably be fair when he finally reaches free agency, actually, but he'd probably take 5 or 6 years at a lower AAV at that point just to lock up the dollars.
There's a huge additional benefit to locking your young guys up to extensions, however. I'd much rather pay Zimmermann 6/$100 or whatever NOW for his age 28-33 seasons (maybe with a few team options years on the end) than be stuck in that trap where we face having to give him a 6/$120 million deal in two years that includes his age 34-35 seasons.
I'd try to avoid as many free agent deals as possible with 30-year old players, who seem to get the same number of years guaranteed as 28-year old players despite being closer to serious decline.