Definitely not his best season. He created 64% of Swansea's goals in 2016/17 with 9 goals, 13 assists & 7 hockey assists.
He also did it despite not scoring one goal in open play with his right foot. He could've finished with 15 goals & assists each that season.
I have zero concern regarding Sigurdsson's age. He will age like fine wine, and I've said that since he was in his early 20's.
Players that decline rapidly after 29-31 are usually much smaller & faster players. Sigurdsson's playstyle is based around his both footed technical ability, not his pace or strength.
Bergkamp, Laudrup & Zidane for an example were all playing at their peak around 35-37. Gylfi relies even less on his pace than those players, and could comfortably play into his 40's.
I expect Sigurdsson to improve with each year, even into his 30's. As long as the team continues to improve his stats will as well.
Sigurdsson has actually directly contributed to 27 of our 54 of our premier league goals this season (50%)
13 goals, 6 assists & 8 hockey assists. Not to mention his 75 key passes.