See, what I got from it was..
Izumi had been in love with the Professor for years, even though he knew it wouldn't go anywhere. He accepted that, because he thought that was enough to make him happy.
He wanted to move on, because we all know how tiring one-sided love can be, but out of fear of the loneliness that would bring.. he kept those feelings close.
The push he needed to finally move one.. was Shizuka. Because now he knew there was more happiness to ne found.. that he also deserves to be loved.
Humans are complex creatures.. we can't really get over even an unrequited love.. unless there's someone who holds our hands and tells us it's going to be okay. Especially so, when one is not self-confident.
So.. Shizuka is not a replacement.
He was just the right person for Izumi to finally understand that he deserves so much more love and happiness.
That's what I think.
very well put
The story was cute but a little rushed? IDK these stories of unrequited love for another person and the cute boy appears and falls in love with that person always makes me think that he will just be a replacement ╥﹏╥