-Or the movie could start with the new suit and we, the audience could intuit that peter made himself a new costume with a new design. It's not a huge leap of faith on the audiences' behalf there. lol.
As it is now, any explanation is akin to: "Oh, yeah, here's X reason (suit gets torn, Gwen designs it, etc...) Peter needs a new costume" when in reality we understand it's the execs saying, "we need to sell more toys". It's just a paper thin facade. It's nothing new, but it's completely lame to claim there's no other way.
But maybe you're right. It's cool if they have the old suit in the beginning and then transition to the new suit. It's keeping in perfect following with the emulation of Nolan's Batman films.