I'm of mixed opinions on this point. First of all, I enjoyed the original Tron a whole lot, and sincerely wished that they had continued it on at the time.
Well, maybe not at the time, since the reason the film didn't really succeed was because its ambitions exceeded what was technically possible at the time. But maybe 10 years later, say, when CGI had advanced somewhat more while still having some of that early unreal flavor.
Legacy, for all its strengths, had to bridge a 30 year gap in which technology had changed. As others have said, you may not agree with their choice, but it was one that they had to make.
Which is why I think it might've worked better not to watch them back to back, since that just throws the differences into relief.
That might work if not for the epilogue of the original, with Flynn ensconced as a happy billionaire flying around in helicopters. It sure didn't look to me like his original, profit-driven priorities had changed in the weeks or months that presumably passed between the Grid and the final scene.
In addition, he and Tron set out to deliberately change the Grid. It wasn't just a tool for users anymore. It was a world that Flynn recognized needed as much nurturing as any other project would require.
The new Grid reflects a change of attitude, both in terms of real world production design, and in-universe character change.
Sure, and within the context of how
Legacy interprets things, that makes sense. But it's not what the makers of the original had in mind. It's not really the same universe they created or would've chosen to explore in sequels at the time. It's a slightly variant universe that incorporates the approximate events of the original but reinterprets them through modern sensibilities.
That being said, CLU's plan seems patently silly in light of the other ways that the program could manipulate the world. Perhaps a better idea would be to create a portal of sorts that allows more access to all the computers in the world. Maybe Flynn had deliberate cut off the Grid, or CLU's portion, to prevent the program's influence from spreading.
Isn't that basically the plot of
Reboot? Megabyte was trapped in Mainframe and wanted to find a way to open a portal to the Supercomputer so he could infect systems throughout the world.