And identifying a WordPress website is very easy by looking at the HTML. Anybody experienced in writing web scrapers has encountered it many times.
That’s what semantic markup is for? No? H1…n:s, article:s, nav:s, footer:s (and microdata even) and all that helps both machines and humans to understand what parts of the content to care about in certain contexts.
Why treat certain CMS:s different when we have the common standard format HTML?
It's simply not possible to carefully craft a scraper for every website on the entire internet.
Whether or not one should scrape all possible websites is a separate question. But if that is one's goal, the one and only practical way is to just consume HTML straight.
WordPress, MediaWiki, and a few other CMSes are worth implementing special support for just so scraping doesn't take so long!
Can you though? Because even big companies rarely manage to do so - as a concrete example, neither Apple nor Mozilla apparently has sufficient resources to produce a reader mode that can reliably find the correct content elements in arbitrary HTML pages.
Maybe I just get your scraper's IP range and start poisoning it with junk instead?
Of course, scrapers should identify themselves and then respect robots.txt.