I think it's a very good interpretation. Just because Opeth has never written political lyrics before does not mean this song can't be political. You can't expect someone to sing about demons and occult forever. People change, and for many people their view on many things change once they become a parent. And I think it should be clear by now, Opeth's music will continue to change.
Besides, it's hardly even a political song, even if it's about GWB. He's an individual who's fucked up the world more than anyone in recent history. Recognition of his incompetency crosses party lines. Any person or politician even remotely informed about current events and history of Dubya's tenure would agree, regardless of political leaning, that he's a total fuckup. This song doesn't have a hidden political ideology, it's just about a great figure who's universally recognized as an incompetent fool, merely in office due to his heirship.
I mean, that's if it is indeed about GWB.
It's obviously a stretch, but I appreciate the thought that went into interpreting the lyrics. It seems un-Opeth to write lyrics about a political figure, but if it was ever done, "Heir Apparent" is how I imagine it would be written; ambiguous and capable of different interpretations. Obviously, Mikael wouldn't write "The Hand That Feeds" type of song and do an interview with MTV..., "I wrote this song about the current US President, who I think is a big screw-up... We have a new guitarist who is among the top 3 in Sweden... Here's the video for "Porcelain Heart".... Remember to buy Watershed, in stores June 3rd!"
Intentional or not, political songs generate some mainstream buzz. There's already enough idiots saying Opeth sold-out. A blunt song with a political message would result in even more people calling them sell-outs. Not to mention there are people out there who think political songs are stupid in general
Anyways, way I see it, the fact that "Heir Apparent"s lyrics spark a heated discussion is a sign of good writing, though it doesn't hurt that it may or may not be about GWB! Mike has written a song that's ambiguous while capable of having decipherable meanings, so it's definitely possible that it's about GWB, religion, or just someone Mike knows for that matter. It allows itself to be seen in different ways, so it all depends on your perspective.