I am 100% behind trying to work young players into positions on the active roster including bullpen and bench roles. I also can appreciate that an older player may fill a need for a limited time, especially in a non-starter role. I expect that Merrifield's role will expand or contract based on how he is doing this season.
I think the Phillies look at him as a potential injury replacement for six starters as well as the more traditional bench role where his starts could give a normal starter a day off. I expect that he will only play second or left field, but Stott could move to short to give Turner a day off, and Marsh could cover center or right field, for a day off for those starters. Merrifield also could start in left, Marsh moving to right, and Castellanos to DH to give Schwarber an off day. Of course, this all fails measurably if Merrifield struggles in the field and with the bat.
As I understand it, the Phillies had already passed into the second tier of the competitive balance levels and even with the Merrifield signing, have plenty of room to make additional moves before hitting the next level, which would involve penalties other than paying the additional tax. While I don't want to see them waste money, they seem to have an owner who is willing to commit crazy money to the team. I think they can still take on payroll in deadline moves if necessary.
Bringing in players in the mid to late thirties remines me of the measurable period following the collapse of September 1964. More often than not, it doesn't really work out.