Yes, he had two years to start in Denver and got worse over time.
After the 2015 Super Bowl seaon, Peyton Manning retired and his back up, Brock Osweiler, signed with Houston because they were willing to pay way more than Denver. So Elway drafted Paxton Lynch in the first round (who was really terrible), but the coach was Gary Kubiak, who favored Siemian, who he had personally selected in the 7th round the year before.
Siemian showed he wasn't the answer, but the next year, 2017, they didn't sign anyone because they hoped that Lynch would be ready. But he was a complete bust and they were again stuck with Siemian, who had regressed. Eventually they did bring back Osweiler off the street (after he'd been traded to and then cut by Cleveland) because the situation between Siemian and Lynch was so bad. All three quarterbacks were gone after the season.
It is what it is. We had a great run with Brees, but we've entered quarterback hell. For a preview of what's likely in front of us, let's look again to Denver. Since Manning retired in 2016, they've had the following starting quarterbacks:
Trevor Siemian
Paxton Lynch
Brock Osweiler
Case Keenum
Joe Flacco
Brandon Allen
Drew Lock
Jeff Driskel
Brett Rypien
Teddy Bridgewater