Selective Availability was turned off in 2000, everybody gets the same GPS signal now. GPS III won't support it, so it won't be able to be turned back on.
Seafloor features will likely last millennia.
Iirc the Chinese are a partner in Galileo (the European program).
This is a good thing, hardware will rapidly appear that supports 2 or more of them meaning that either all the partners would simultaneously have to agree to disable theirs or (more unlikely) someone was shooting down the satellites which since that would realistically only happen during a global conflict means I'd have bigger things to worry about at the time.
Has been out for years. E.g. iPhone 4S supported GPS and GLONASS.
Now we have a system where the chinese stopped being a partner, and the US forced us to add a backdoor and reduce the quality. I’m not sure I can see anything good in this.
Jamming is a local action, the US will be able to jzm Gallileo like we are able to jam GPS - again locally.
Frequencies can be switched, if we only relied on the fact that they are the same then the US could switch theirs (and have equipment which is compatible if course) and there goes our security.
I work in security and have worked for 12 years for a huge provider of telecom systems and hinestly do not see any real advantages to match the frequencies.
Also, if we had the same frequency, WE could not jam GPS if we wanted to.