There's not much to gain by making it compatible with RSS because the existing toolset and ecosystem around RSS are for publishing and subscribing to content, whereas Bitfeed is for Bitcoin transactions. Simply using an RSS format won't make Bitfeed automatically render in RSS readers because they're simply transaction IDs and raw transactions.
XML is not really a good idea in 2020 either. The case for using JSON over XML is demonstrated by projects such as JSONFeed https://www.jsonfeed.org/2017/05/17/announcing-json-feed.html
Overall, there's not much value gained and unnecessary drawback by using XML/RSS.
We build this from scratch to optimize for Bitcoin. RSS will be used as an inspiration for future design decisions.
I agree about JSON vs. XML (i despise XML) but AFAIK this JSON spec has hardly any adoption by sites who maintain RSS feeds. Also it is trivial to transform XML to JSON (JavaScript based parsers do this automatically).
The benefit I see is that all existing RSS parsers / technology could be used and then add the Bitcoin stuff.
It's trivial to transform JSON to XML, as well. If you wanted to make a platform to transform Bitfeed into RSS that'd be fairly easy, but IMO it's a second layer protocol at best, and I think the end game of "replicating RSS" really misses a lot of the value add of Bitfeed