Parsons is not a top tier player. He is a mid tier player getting paid like a top tier player. But that is the market. If Rudy Gay hit free agency today, he would be getting paid a max level contract because he's a 20 ppg player who has shown in his career that he is a good starting SF. That is not to say he is worth a max, likewise Parsons getting paid a max doesn't mean he's worth it, that is simply the market they are in. If the Grizzlies didn't pay him, someone else will.
What other choice did they have?
They are built to "win now". Now I'm sure we would agree that it is unlikely they will win a championship, but they are built to compete, and their existing roster structure dictates they have to go after veterans to help maintain this level of competitiveness. Otherwise, they need to blow it all up and trade Randolph and Gasol, not re-sign Conley, and not bring in Parsons. At that point they need to tank. Unfortunately, their 2017 first round pick is owed to Denver (top five protected), same protection goes to their 2018 first round pick, and then it becomes unprotected in 2019. After which, they owe another first round pick to Boston which can be traded as early as 2019 (top 8 protected) or 2020 (top 6 protected), or 2021 (unprotected). So in a best case scenario they keep their 2017 and 2018 draft picks, trade their 2019 draft pick, and then potentially lose their 2020 pick. It is not really the best situation to be in for a rebuild, unless of course they can go into asset acquisition mode like the Celtics.
As for their players. Ok, Tony Allen and Zac Randolph are 34 years old and near the end of their careers. But both guys are potentially replaceable by players they have taken in this year's draft: Wade Baldwin (combo guard) and Deyonta Davis (PF/C). Both players have a ton of upside and could feasibly slot into their starting SG and PF slots in the near future. Conley and Parsons are entering their prime years. And while Gasol is 31 years old, his style of play should allow him to remain a very effective player for quite some time because his game is not built around top tier athleticism like Whiteside, it is built more on touch, finesse and smart defensive ability. He should age well as long as he comes back from his injury. Also, building this way they may lose that 2017 draft pick if they qualify for the play offs, and their 2019 may head to Boston if they qualify for the play offs, but they would have 2018 and 2020 onwards to use to bring youth in. Likewise, they have an extra second round pick next year; one more in 2018, and two more in 2019; all of which can be used to get more youth.
At the end of the day - Conley / Allen / Parsons / Randolph / Gasol - should keep them competitive for a season or two. Then they can swap in Baldwin and Davis when needed, or as the cap continues to go up, look for other free agents to come join them, and they'll get more cap space because Randolph is out of contract next year. So they have a good competitive veteran line up, some talented youth, and enough space and assets to continue to keep themselves competitive for the foreseeable future.