Why RNAV SIDs and STARs are RNAV 1 instead of RNP 1?
First of all, an RNP 1 procedure requires a GPS/ GNSS. And as you have said, it requires an inbuilt navigation monitoring and an alerting system. On the other hand, an RNAV 1 procedure does not. In RNAV 1 you can get navigational aid from the IRS/ VOR/ DME or IRS/ DME/ DME.
The SIDs and STARs can either be RNAV 1 or RNP 1. RNAV 1 is the system introduced at the start of performance based navigation to cater for smaller air spaces (terminal area procedures) and it is a step towards changing over to RNP 1. Many countries these days require an aircraft with RNP 1 capability to fly their SIDs and STARs. They sometimes tend to name it unambiguously on the chart. Like a chart might state, RNAV 1 and then state that you require a GNSS. This essentially makes the procedure an RNP 1 procedure. Others state specifically that you need to be RNP 1 complaint. This means that you need to have a GPS and a monitoring and alerting system. This is mainly seen in China, where most of the airports require the aircraft to be able to fly RNP 1 procedures so that they can pack airplanes together, with minimal ATS surveillance. This is done to accommodate the ever increasing air traffic in the region.
It has been quite a while, since I have not been to Wuhan, China for obvious reasons. Wuhan airport is one those example airports which allow only aircraft that can fly RNP 1 for their SIDs and STARs.

I think the advantage of keeping the charts and procedures RNAV is that it allows operation of older aircraft, which does not comply with RNP operations. This is I believe quite important in a place like the United States where airlines still tend to operate older aircraft and has a big general aviation sector. In other places, airlines tend to have newer aircraft, which easily fits into the current navigation standards. This makes it easy for the regulators to change their procedures to RNP.
For the question regarding losing RAIM (Receiver autonomous integrity monitoring). If you lose RAIM, you probably cannot do a GPS/ RNAV (GNSS)/ RNP approach (same type of approach, just different names). But the aircraft can still be used for RNAV navigation, as RNAV does not require the monitoring and alerting.