My concern with extending Burnes is not so much his age, he’s got a lot going for him that should allow him to age well. He’s not overly reliant on velocity and has really great command, elite ability for spin, and could lean into throwing more sinkers/curves/sliders and fewer cutters than he does now. He also been a really durable SP his whole career. He checks a lot of boxes, even if he’s unlikely to ever be the 7 WAR pitcher he was once.
Pitchers like Burnes can remain really effective into their mid-30s.
The concern is just how much he’ll cost, for how long and the potential for injury for any pitcher. Pretty good chance he finishes in the Cy Young running this year and is going to want something around Gerrit Cole’s deal. I don’t think he signs an extension that doesn’t exceed that anyway, so he’s really going to just be a free agency signing. Burnes blowing out his arm on a $300M deal (or even “just” a $200M deal, which I think is the floor of what he’ll get if he performs as expected this year) could be pretty crippling to the Orioles, and I have a hard time believing the Orioles are going to outbid the big markets clubs for him.