I see your point, but I think Fournier addition, along with Horford and healthy Rob is a recipe for a very competitive team. Plus how can you argue Evan taking minutes away from them. Doesn't Richardson create same issue? Personally, I would have favored playing the two J's, Fournier and one of Nesmith and Romeo (the other coming off bench) along with a center rotation of Rob and Al. Tatum seems better suited to play power forward in my opinion. Water under the bridge.
Fournier and Richardson do pull from the same pool of minutes, for sure. There is a difference in that you would be giving Fournier starter minutes (30+ in my mind) where Richardson's role appears to be bench so probably 20-24 min. That is not a huge difference but a little difference.
And the other thing is that even though the vast majority of the people on this forum seem to want to start 3 wings and one big, with Tatum being the "kind of " PF, it does not appear that the Celtics share that view. So if the starters or core unit are Smart, Brown, Tatum, and some combination of two bigs, Fournier is a bench player. The most used line up for the Celtics for 5 of the last 6 seasons has been a two big line up, including last season (Theis and Thompson).
I think this all contributed to the decision to not sign Fournier and to take on two players, Richardson and Schroder, who are more suited to 20'ish minute bench roles. Eventually, they will find a starter level PF to add to the team. Until then, I guess we get by with Grant Williams in that role.