2. Age and fitness

Partey may have been consistent with his performances for Arsenal, but he was not so constant when it came to his fitness. During his five seasons at the club, he missed 79 games with injury (including international games in those periods), a considerable amount for a player who was relied on for good performances.
While his availability was far better during last season, playing 52 times across all competitions, it's unlikely that would have remained the same going into the new campaign. Partey has recently turned 32 and as he ages, his fitness is likely to slowly deteriorate.
Tying him down to a new contract when his fitness and availability was likely to go downhill would not have been smart. Arsenal wouldn't have wanted to be paying out for a player who may not have been available for enough of the season and so it made sense to let him leave.
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