They absolutely could, and I'd guess UC has a laundry list of replacements ready to go into them (and Toon Lagoon, and all of Lost Continent, and Marvel Superhero Island) just as soon as someone in charge decides it's a necessity or makes business sense. Until then they'll sit empty and maybe occasionally get used for special events.
It's no different than Disney leaving things unused or under used, but at Magic Kingdom that makes ZERO sense since they have higher attendance numbers and need all the attraction capacity they can get.
Now if suddenly Universal's attendance shot up by a couple of million guests a year and they REALLY needed the capacity, they would probably begin demolition and build new attractions in their place tomorrow.