Everyone seems to be discussing whether Bumblebee is a reboot or not. This question can only be asked based on one basic assumption; that a continuity exists for it to be, or not to be, a part of.
On one hand, Travis Knight says there is a continuity and he had to follow certain plot points. Michael Bay said all of his films were sequels. Fantastic. On the other hand, take a look at the
actual continuity of the Bayverse films. Megatron's dead at the end of the 3rd movie,
very specifically dead in the 4th and comes back as Galvatron, and is then inexplicably back as Megatron for the 5th.
Now, while that's just one example of the many, many continuity plotholes the Bayverse has I bring it up for a specific reason: They stopped Megatron's inclusion in a prequel due to "continuity" while previously allowing Megatron to appear in a sequel
despite "continuity".
What they
say doesn't match what they
do.
If you prefer Bumblebee to be a standalone movie, that's your choice. If you want it to be a prequel, go for it. If you want it to be the start of a reboot, that's fine.
Fortunately for Transformers fans we have the Unicron Singularity for just such situations.