If Grandma had to install a seperate app to use RCS, she too would probably end up using Signal, since the barrier to entry is the same.
The reason iMessage is popular in the US is the fact that it's functionally just SMS, being used by the default message app. The reason that didn't happen in Europe is that SMS used to cost money to send, so nobody was already deeply invested in that system, but instead rushed to Whatsapp et al., since those were free and SMS was not. SMSes are free nowadays, but by now we're all already invested, and all the apps provide a better experience than SMS and RCS (the former due to lack of features, the latter because its often broken) and even Grandma has Whatsapp to keep in touch with family, if only because little Timmy installed it.
The reason iMessage is popular in the US is the fact that it's functionally just SMS, being used by the default message app. The reason that didn't happen in Europe is that SMS used to cost money to send, so nobody was already deeply invested in that system, but instead rushed to Whatsapp et al., since those were free and SMS was not. SMSes are free nowadays, but by now we're all already invested, and all the apps provide a better experience than SMS and RCS (the former due to lack of features, the latter because its often broken) and even Grandma has Whatsapp to keep in touch with family, if only because little Timmy installed it.