The removal of I-345 in Dallas would put all freeway traffic towards the I-30/I-35E mixmaster and make a busy interchange into an even worse chokepoint. Even one wreck in the Canyon, Mixmaster, or along Woodall Rodgers would make commuting in any part of the remaining freeways a messy nightmare. The side streets in Dallas are already way *way* behind in keeping up with potholes as it is, and adding more non-essential traffic to them would really exacerbate things in/near downtown. Retaining what is there may not 'look' good to some, but at least there are ways around traffic problems that would arise in that area. Putting I-345 (eventual I-45 extension, maybe??) below ground level would placate at least some of those who are into highway corridor aesthetics, and add a potential extra possible deck park opportunity for those who are fans of Klyde Warren Park (the new deck park over Woodall Rodgers).
The latest City Council recommendation made about the proposed redo of I-30 in and east of downtown, would ask TxDOT to work simultaneously on I-30 and whatever would get done to I-345. Another proposal someone hasn't thought very long on... whoever thinks that the State will agree to do both freeways at once, doesn't realize the traffic headache that would result from having 2 freeways (that cross!!) out of commission partially or completely. TxDOT hasn't answered the City's counter-proposal just yet, but I doubt that they'd agree to redo both freeways together.
Either way, there isn't a whole lot that would be positively gained by completely removing I-345. If anything should be removed regarding it, it should be the numbering. No one calls it that, and it's really too short for a legitimate separate number. Either US 75 should end at I-30, or I-45 should end north of I-30 -- many miles north of I-30 would be even better.