With Griezmann you have to consider more variables.
1. Suarez won't last forever. I doubt he can maintain this level for more than one more season so most probably he won't be our starter in 2019/20
Griezmann is 4 years younger than Suarez, usually when you need a replacement of an aging player you want someone who can give you longer, 27 isn't old but also isn't long term replacement, I honestly can't see him being a player who retain his for into his 30's like Messi or CR7 (up until last season)
2. Price of 100 mil is huge but it's more than fair in today's market for a player of his level.
This is just assumption, 100M is the price for AM , but when players has such leverage they knew it and they want the extra money for themselves. Sanchez was supposed to be cheap deal with 6 months left and at the end he got huge salary + huge signing bonus etc.
Who says Griezmann won't be same case?
Also there is a question about our budget, fair price doesn't equal great deal. We have to look at Opportunity cost. Well Griezmann deal affect signing other players? would it stop us going after De Ligt and Arthur for example? And do the club really believe that those two (or others) the real deal?
So if you take these two facts into consideration things start to make sense in the long term. We will have to rotate heavily just for one season but after that Suarez will most probably end up as a backup.
In 2018/19 season Griezmann would be 27, Coutinho 26, Dembele 21. If you add Messi to them you are set to go for the next 4 years. That's called a smooth transition between generations and we would be among very few who were able to do that in football history. If the price for that is one year of heavy rotation I would sign that immediately.
I am not sure how he is long term with his style of play and age? We will need someone younger tbh, we are right now gonna face a huge issue when generation 87 start declining badly, Messi , Alba,Rakitic, Busquets,Pique, Suarez all born between 87 & 89 so we will need a lot of reinforcement to replace them , I won't prefer doing that by just adding players who are 4 years or so younger. Let's have players who are actually of different ages to allow smoother transitions as you say