Coaches make changes in the last quarter all the time. If you're 25 points up it's entirely sensible to take off key players. What the coaches can't predict, however, is the replacements coming off the bench with no intensity, or even brain function when it comes to making even competent decision. That is entirely on the players.
Thomas needs to be introduced at some point before the end of the season. What would be the use of waiting until we get to the Challenge Cup final (Lord willing) and then have to start him with no game time if for whatever reason injuries had decimated our back row? It's not like that hasn't happened before.