It's great that Tom Welling is returning as Clark Kent aka Superman in the Arrowverse's upcoming Crisis on Infinite Earths crossover event, but in doing so, the producers risk ruining Smallville's perfect ending for the character and his journey. Rather than focus on a superhero in his prime, Smallville was about Clark becoming Superman; he fought many of the villains that he would have as the Man of Steel, but he instead did so as the Blur, completing a set of trials.
Everything that he encountered in his high school years and then again while working at the Daily Planet prepared him for his ultimate showdown with Darkseid in Smallville season 10, saving Earth from Apokolips. What's more, he came to terms with his friend and arch-enemy Lex Luthor while also getting to see his father one last time, who presented him with his iconic Superman suit. In doing all this, Clark's journey to becoming Superman ended and his life began anew - but Arrowverse risks tarnishing that with Crisis on Infinite Earths.
A lot happened to Clark Kent after Smallville ended - much of which has been explored in the Smallville season 11 comics, which have been confirmed to be canon in the Arrowverse. But with Tom Welling returning as Clark in Crisis on Infinite Earths, fans will now get to see him as Superman - a full-fledged incarnation of the comic book superhero. It's a great continuation of his story - from an evolution standpoint - but it's a bad idea on a conceptual level because it goes against what Smallville stood for: Clark Kent, not Superman.
Fans of Smallville know the famed "no tights, no flights" rule that the writers and producers lived by throughout the show's ten seasons (with one momentary exception of the flights part of the rule in one of the earlier seasons). This is part of the reason viewers didn't actually get to see Clark in the full Superman suit in the series finale, despite the episode showing a silhouetted CGI version flying in the sky. So putting Welling in the Superman suit and having him fully fight the Anti-Monitor in all of his Superman glory means going against Smallville's core principles.
Beyond that, though, being innocent to Clark and Lois Lane's lives after the series finale, including everything that happened in the seven-year time jump, was perfect. There's a sort of ignorance to the future that made not seeing Clark as Superman truly worthwhile in the end. But Crisis on Infinite Earths throws all of that out the window. It's like a movie coming back for another round after a perfect ending, just because they can. Sure if anything were to bring Welling back into the role of Clark Kent, Crisis on Infinite Earths is it. It'll undoubtedly be exciting to see him back in the role after all these years - but that doesn't mean his appearance won't have unfavorable consequences.