Boise has been a powerhouse in Div1-AA. They moved up to Div.1-A and they have only played in Div.1-A for 15 years and they are already the top mid-major in the country. The have won 2 BCS bowls, have had 2 undefeated seasons, have victories over TCU, Iowa State, Oklahoma, UTAH, ASU, and Arizona in bowl games. I am pretty sure BSU Alumni are licking their chops to get into the Big 12. BSU isn't thinking they are going to get hammered for a few seasons, they are thinking they will compete in the Big 12 right away. BSU wouldn't think twice about staying in the Mountain West if offered an invite to the Big 12. Big 12 vs. Mountain West.....uuuuum thats a no brainer. Boise would do very well in the Big 12 and everyone in Boise knows it.
When they moved from Div.1-AA to Div.1-A they weren't thinking ohh we will get hammered for awhile. They saw it as a opportunity to move up and took advantage of it. They will think the same if offered an invite to the Big 12.
I'm sure they're not "licking their chops" to get into the Big 12. You forget, or left out, Boise had an opportunity to leave for the Big East (geez), and changed their mind for a variety of reasons - namely, the MWC ponied up more money to keep the Broncos in the conference to the detriment of other member schools. Somehow people think Boise would be willing to give up the power for more money and less influence. Who does that? Of course Boise wouldn't willfully take a back seat in the Big 12, but they're also not coming in and calling the shots either.
Lastly, you listed the bowl games they won. Yup, they did, but beating a team you face once - is a lot different than playing that same team annually and beating them. Hence, getting thumped for a bit before becoming competitive. See Utah joining the Pac 12 for an example.