To be fair, he was a productive pro before coming to the Senators. He was PPG in the AHL as a 20 year old and ended up on San Jose right away, then he was eventually a top 5 scorer on San Jose.
Getting traded to Ottawa was probably the worst thing to happen to him, because they were never going to give him term after his bridge deal was up.
Both Tierney and Brown had really bad luck with both being on Ottawa when they were ready for their big payday, and with their payday coming in 2020 when COVID restricted the cap. Ottawa wasn't giving anybody term except for the top young stars (Chabot, White, Tkachuk, etc).
Had both of them been on a different team, there is a chance that they would have signed a longer term deal a year early before COVID hit, because they were still seen as productive players relative to their role.
He probably would have been bought out a few years down the line, but still - going to Ottawa probably cost him millions of dollars.