Yes it was, originally the word dinosaur was invented in the 1800s to refer to those ancient “big lizards” and birds were not included that definition……..
In case you haven't notice, we learned quite a few things since the 1800s.
It's called "progress".
my only point is that the question on how dinosaurs evolved in to birds is not solved by simply changing the definition of dinosaur and say “ohh but birds are dinosaurs”
The changed (aka "improved") understanding of what dinosaurs are was not some arbitrary thing like you seem to be insinuating.
It was instead necessitated based on new discoveries. In case you didn't notice, we discovered quite a few more dino fossils since then. Our understanding of evolution, anatomy, fossils, genetics, comparative anatomy, etc etc etc etc, also improved quite a bit since then.
It's almost like you are actively trying to ignore 200 years worth of scientific progress.
My only objection is that there is no conclusive evidence that they evolved trough the specific mechanism random mutation + natural selection.
That's a bit of a nutshell way of stating it, but true nonetheless.
There are no other known mechanisms and the ones that are known, are sufficient to explain the facts.
If you think there are other mechanisms that would need to be included, you are free to suggest them + provide the evidence that you feel justifies the need to include them.
If you can't do that, then it seems all you have is this argument from incredulity.