Dylan O' Brien has the acting ability - he's capable, talented and could hold his own in the MCU. He just happens to have the looks for Peter too.
If Spidey is gonna be in the MCU, he needs a likeable actor who's good at their job but isn't so amazing that it'll be impossible to recast.
A good example is Hemsworth - he's good at acting, he looks the part... but it won't be difficult to find somebody else who looks like Thor. The look is so generic! Big, blonde, macho man. Peter Parker is a skinny, average guy with brown hair. SO MANY OF THESE PEOPLE EXIST.
Instead of looking for an RDJ, a Chris Evans or a Hiddleston - look for a Hemsworth.