Changing ANY other interface name does NOT have the same effect. It bothers because it's unnecessary to disconnect from a technical point of view and is an inconvenience to user. PPPoE connections also don't establish as fast...but regardless why disconnect on a simple name change when it is possible not to?That "arbitrary name" as you call it, is also used in other places in RouterOS and some stuff depend on it (firewall rules or whatever). They can't work using some "name" that doesn't exist (if you change it).
Just my two cents on this.
Why does it bother you?
Why? what is your reason for that? Does the name of PPPoE service have to be sent to provider? if not, how is it different from any other interface name?With a PPPoE user, I will agree with way it is, if anything changed, restart service.
What is technically wrong is wrong no matter how you spin it. I would write about me changing an interface many times if that was an issue but it is not. Issue is explained really well and it IS AN ISSUE.How often do you change your wan interface name that this is such a big issue for you?