what are you talking about? "I" was number 1 on the charts and one of Taeyeon's best digitally performing songs. If anything, her consistency has been a lot more erratic in the last 2 years (hasn't really had a hit since "four seasons" and even before that there were up and downs like "fine" vs "something new"). Her album sales have also stayed relatively stable over the last 5 years - she sold over 100k for her first mini-album and sold a similar number for her most recent mini album (her full albums sell better by about ~50k).
It took IU 2 years to get big (she debuted Sept 2008 with abysmal digital/physical sales, "Good Day" came out Dec 2010 and that mini sold over 90k) but you can't compare at all because IU didn't start in a group - when she debuted as a solo, she started at 0, with zero recognition, fans, hype, or reputation.
The main problem with Wendy is lack of promotion and hype. Has she been on a single radio or promotional show at all? Did SM just release her album and then peace out?
I think there could be an argument made for Spark being a hit and All About You was most definitely a hit, but yeah overall she's never been super consistent like IU, but she always garners the public's attention regardless if the song sticks or not