Because it would give validation to the correct line of thinking in the future, and validation to all those people who worked to make it happen.
Firstly, it was never about supporting the artists financially, that work has already been done. VFX artists are salary workers, they don't earn portions of proceeds. The money you spend seeing any movie will always go to the producers regardless of if it's good or bad. At the very least, supporting the movie financially validates their work. No artist, no person, ever sets out to make a bad movie. Showing those artists that their work was appreciated is a major thing in its own right because those positions are often labor intensive and extremely thankless.
Secondly, I know your qualm is that the executives made a mistake initially, but the important aspect of this entire circumstance (and why the Sonic movie will likely be remembered and important for the future of Hollywood) is that this is probably one of the very first instances where a big executive studio actually capitulated to audience response. Hollywood is a stubborn industry. No matter how dumb, stupid, or inane an idea is, the norm is that big movie studios will almost always stick with that decision. Just look at Cats for instance. The public response to that film was also highly negative on the basis of the visual designs, and the studio involved had no interest in doing anything to change that, and the movie suffered because of it.
For a time, it was looking like the Sonic movie was going to go the same route. The movie would come out with Sonic the Manhog and flop and yet another video game movie would be down the drain. The thing with previously-established franchises in Hollywood is that most executives don't give a hoot about source material or faithfulness or pleasing the fanbase. With the Sonic movie specifically, the aim was never to make a movie to please Sonic fans. On Paramount's part it was to cash in on a big-name character and on SEGA's it was to draw in unfamiliar people to the IP. Because of this, the executives put a stranglehold on the creative process. If you think the original design was all a part of Jeff Fowler's vision, I'm sorry to say that's just incorrect. His initial idea was much closer to what we have now, but Paramount tied him down because the studio is the final say. The negative backlash from the original trailer was the fuel Fowler needed to convince the Paramount suits that their idea was going to negatively impact the final result, and because of that they let loose on the reins.
And we have a better movie for everyone because of that. Regardless of your opinion on it, I think it's nearly universally agreed that this version of the Sonic movie is a better overall product than what we would have received.
So when we're voting with our money on this movie, yes, you're giving your money to Paramount, the guys who messed this up in the first place. But what we're validating isn't their original dumb idea, we're validating the approach of pleasing your audience. Look at Bumblebee and how that movie, with its very obvious respect for the franchise, completely revitalized interest in Transformers movies. Look at Legendary's new Godzilla franchise and how its heavier influence from the origins of the franchise made Godzilla the best he's ever been in the West. Look at how the DCEU is looking to completely reboot itself to be more faithful to the comics.
I really do think we're seeing the beginning of an age where these kinds of movies based on established franchises are starting to open up to the public's wants. We're seeing less and less studios enforcing their rules to the bitter end. We saw it when comic book movies came into prominence, we're seeing it now with the superhero movie post-Dark Knight.
I completely understand your lack of interest in this film, and I'm not trying to change your opinion. Some things just aren't for some people, and I'm more than okay with that. What I am trying to do is at least try and convince people who are apprehensive to keep an open mind for the future and not completely write-off something that can have a great degree of significance for cinema going forward.