#3 Lionel Messi will be playing with a new set of players

For a player who has spent his entire professional career at one club, making a jump at 34 is not going to be easy. The circumstances have forced Messi to leave Barcelona and he will need to unlearn quite a lot in the coming months.
The possession-based game that Barcelona play is quite different to the style that Messi's potential suitors, Paris Saint-Germain, have adopted. It's not a far cry from Barcelona's but with an entirely different set of players around him, Messi will need some time to fully settle down into his environment.
It will take him a while to be on the same page as his new teammates. Messi will need some time to get used to playing in Mauricio Pochettino's system as well.
#2 Fatigue will catch up after a gruelling season

Cristiano Ronaldo and Lionel Messi featured extensively in the 2020-21 season. After the Covid-19 recess, fixtures came thick and fast for footballers and they did not get a proper break between the seasons.
The 2020-21 season was pretty congested and Cristiano Ronaldo played at Euro 2020 while Messi represented Argentina at the Copa America this summer. After a short break, Ronaldo has had to report for pre-season preparations.
Messi is likely to join a new club soon and will be dropped straight back into the deep-end. Ronaldo is 36 now and Messi is 34. There have already been a few indications of their age catching up with them.
Living on the wrong side of 30, both players could be affected by fatigue as they have played an incredible amount of football this past year.
#1 New stars like Haaland and Mbappe will take over

Erling Haaland and Kylian Mbappe are viewed as direct successors to Cristiano Ronaldo and Lionel Messi. Both individuals have clocked incredible numbers in recent seasons. They are expected to kick on in the same vein as they seem well settled at their current clubs.
Haaland scored 41 goals and provided 12 assists from 41 appearances across all competitions for Borussia Dortmund this past term. Mbappe scored 42 goals and provided 11 assists from 47 appearances across all competitions for Paris Saint-Germain in the 2020-21 season.
Erling Haaland turned 21 in July and Kylian Mbappe is 22-years-old. This will undoubtedly be their year and both could easily overshadow both Messi and Ronaldo in the 2021-22 season.