1. You know I've heard that stupid Sam Smith on the radio for a while now and kept thinking isn't this just a crap version of something I've heard before? Thank-you Tom Petty for reminding me you got there first! Or at least before Sam Smith anyway.
2. This argument that all the chord/note combos have been used is about as clever as saying "I've seen all the colours known to man, how can an artist ever make an original painting!?!?!" Or "Oh no we've used all the words in the dictionary, how can make an original book!?!"
What makes songs different it not just the chord combinations, but the tempos, the notes in the melodies, the beats, what instruments are used.
Sure you might get some overlap of chord progressions being similar, but when pretty much the whole song sounds the same it’s pretty much odds on you can cough the word plagiarist. Loudly.
Ultimately Sam Smith himself is probably too musically clueless to have heard Tom Petty's original and the similarity is down to whatever major label hack songwriter/producer he was working with thinking they could borrow from Petty and nobody would notice.
It’s also down to the utterly cynical way in which major labels write music. I understand the suits have got musical experts to analysis what chords have tended to make hits and then they order their songwriters to recycle those chord patterns ad infinitum meaning so much of today’s music sounds like such a samey mush.