It'd take several years at best to field a Shuttle replacement. Even in the 1960s, it took several years from beginning of a manned space vehicle program until first flight. Given lower funding today, it'd likely take even longer. The last Apollo flight happened in 1975 and the first Shuttle flight wasn't until 1981. Even then, the Shuttle had been in development for several years before the Apollo program ended.

Without the Shuttle, we'd be left with maintaining the ISS using only Soyuz and Progress vehicles (unless we could get the Chinese to help). Those vehicles just don't have the capacity to keep a crew on the station and maintain the necessary logistics for a long period of time. We might be forced to put the station into hibernation, perhaps with periodic maintenance visits. Stuff breaks in space, and so would an unattended ISS.

Some would argue that getting rid of both the Shuttle and ISS would be a good thing. After all, the Shuttle and ISS have sucked up most of the NASA funding that could be used for other, sexier missions. Personally, I doubt it. If we end up writing off the ISS, I think it'd be a very difficult sell to get Congress to fund anything else. Sad to say it, but that could well end the US manned space program.

Should we replace the Shuttle? Yes. But dumping the Shuttle without a program to develop a replacement in the works just doesn't seem like a good idea. There are too many in Congress who'd use the opportunity to rip into NASA's budget to spend it on other things.