Here's another reason why Glover won't work and it's nothing to do with his race. It's his age. The reboot is supposed to be Ultimate Spider-Man, meaning high school Spider-Man. There are some actors, Nicholas D'Agosto and Aaron Yoo for example, who are in their 30's but still look like they could walk into a high school and pass for students. Donald Glover isn't one of them. He's in his late 20s and he looks it.
And before anyone comes at me about Tobey being 27 when the first movie, not only did Tobey look younger, but he was playing a senior that was about to graduate. A bit of a difference from someone who's nearly 30 and looks like it playing a 15 year old freshman/sophomore.
Glover actually looks his age and what about in 9 or 10 years? Look how old and frumpy Tobey started to look and people started to whine that he was too old. Casting another guy who'll actually be older than Tobey was when he started is doing the same thing.