As @FlaBuff mentioned this has been common for a long time. With the additional weeks of practices many coaches worked in time to start preparing guys for the next year, sometimes even starting to put in completely different systems.I honestly think the death knell for bowls started long ago. Since I've been following CFB, many coaches have treated bowl games as exhibitions: relaxing player rules (e.g. curfews, practices) and giving practice snaps to next year's starters vs the current years.
The players opting out though is now much more common.
This again can be traced back to the impacts of the playoff. Teams used to focus their goals on a specific bowl game, in the old PAC8/10 and the B1G from the first day of spring ball the players were focused on Rose Bowl, Big 8/12 teams know that a successful season meant playing in the Orange Bowl, etc.
Even for lower level leagues like the old WAC (now basically the MWC) they knew they weren't going to be in the conversation about the national championship but they knew that if they won their league they could play for the Holiday Bowl trophy so that was what they focused on.
Under the current system the focus is on being one of the four playoff teams. Any other result means that you fell short of your goals and the bowl game is a reward more that something to focus on winning. For the G5 teams the focus is on getting the NY6 slot (Cincinnati this year) again with everyone else falling short of their goals.