^^^
Of course, if people claim to find it confusing to have the minuscule route in DC bearing the I-695 number—a claim I think is ridiculous, of course—then surely it would be far more confusing to them to have a considerably longer route bearing that number. I could perhaps see some greater legitimacy to the claim if I-695 ran from the Springfield Interchange up Shirley Highway and the Southwest–Southeast Freeway to the 11th Street Bridge. I still don't think it would be as big a problem as some people would make it out to be, but I recognize there would be a lot more people claiming confusion.
If it were I-695 from Springfield across D.C. (and maybe including what is now I-295 in D.C. and Maryland), then you have two Maryland I-695's, and a potential for confusion for out-of-town people thinking that I-695 in Springfield is the same road as I-695 around Baltimore.
I assumed WillWeaverRVA was referring solely to the existing I-395 and I-695, not to I-295, because the comment from Roadsguy to which he was responding referred to leaving I-295 and DC-295 alone. Otherwise, I would completely agree with you—changing the existing I-295 in DC to I-695 wouldn't work for the same reason why my brain fart–induced comment about extending I-295 down I-395 to the Springfield Interchange wouldn't work, as vdeane correctly noted.
In other words, I was assuming WillWeaverRVA meant I-295 and DC-295 would stay as they are, but I-395 would be truncated to include solely the Center Leg Freeway (more commonly known as the Third Street Tunnel) and I-695 would be extended down to Springfield to cover the rest of what has long been I-395. I tend to think that scenario would provoke more confusion, even though the putative I-695 wouldn't enter Maryland, and I also think I-395 is so well-entrenched in the DC area (having been here since the 1970s) that it would be counterproductive to change the Virginia portion.
Edited to add: froggie makes a valid comment as well about the existence of a secondary route 695 in Fairfax County, though I daresay that number is not well-known and would not itself be a reason for widespread confusion. Off the top of my head, now that the Fairfax County Parkway is no longer Route 7100, the only secondary route number I routinely hear used is Route 644, which the traffic reporters refer to that way in no small part because it changes names when it crosses I-95 (and it's also likely faster to say "644" than "Old Keene Mill Road" even though they're the same number of syllables). Of course there are BGSs that list secondary route numbers (Van Dorn Street, Telegraph Road, Braddock Road, and Gallows Road all come to mind on the Beltway, as does Edsall Road on I-395), but that doesn't mean area residents ever use those numbers. As you know, my neighborhood is near Van Dorn Street and I have never used "Route 613" in giving directions in the almost 20 years I've lived in my house.