This is a thing I saw before on another thread in AW, and I don't understand it. If the books are still published and sold, in several countries, now and for the latest 100+years, it means they are interesting, immortal and they are still read, so... what's wrong with comparing with classics? So what if it isn't current? If people don't write it now, but the genre is liked and read... I am going to write it and give them more stories in the style of Alexandre Dumas or Raffael Sabbatini! (Yes, the literary critics compared my first 2 novels with them, as well as with less known classical authors of historical adventures for teens).
There are a bunch of things involved in this.
-- Just because something is still read and sold doesn't necessarily mean it's popular, especially with the target audience. I'd wager a shitton of copies of Catcher in the Rye, Tess of the D'urbervilles, The Scarlet Letter, etc., are sold to people who have 0 interest in them outside of the assignment the people who buy them are reading them for.
-- Even if everyone reading classics is doing so because they love them (which obviously isn't the case but if it were), that doesn't mean the same thing would sell if it were newly marketed today. There're all sorts of things wrapped up in people loving classic stuff, not the least of which is nostalgia. Like, the 80s movie Overboard is loved by many. It airs all the freaking time. However, when they went to remake it, they had to change things so it wasn't about a guy basically kidnapping a gaslighting a woman with no plans to do anything but keep her as a slave. If someone pitched the original today, it'd not get made. Someone saying your published thing reminds them of old work they love is a compliment, but it doesn't mean that's where people should start writing, with that idea.
If someone tried to shop the equivalent to LotR today, they'd have a problem. Not because people don't genuinely love LotR, but because it's a thing unto itself and another book now that tried for 100 pages of walking isn't what most readers want. Even if they want it in the form they read as kids, because....
-- Classics are often classics for a reason. Most people, I dare say, aren't Tolkein. Most people aren't a Bronte sister. The combination of writing talent, storytelling, inventiveness, etc., that combined to make the books we now consider classics stand out among the thousands of books of the same era isn't easy to reproduce. It's also kind of uncouth to have the hubris to compare yourself to someone of such renown.
-- In a query and construction sense, if you don't know the current market, you're likely in trouble. Yes, it's possible you're the rare person who writes something no one has written for a long time, very well, but using non-current works as comparisons just signals you may have no idea what's currently selling, to editors or readers. You might say, 'it doesn't matter what's currently selling, because people love LotR and...' except first, see the point above, second, the GENERAL population of readers' tastes change over time. This is reflected in what sells today. If you're writing a YA novel but haven't read one published in the last couple decades, you probably don't know what CURRENT readers of the genre expect, in terms of voice, content, etc. You also don't know what's been done to death. If the last book you read was Hunger Games, you might think 'oh, a dystopian world but with...' and then wonder why you can't get anyone interested in it (because dystopian is dead atm; it got beyond saturated).
Agents won't love it if it's a saturated or recently-saturated genre or trope, they'll just pass, because editors don't want it, even if it's good, because they've decided not to chance any more for the forseeable future. It's a business.
Writing to 'I know some people who love LotR and want more just like it,' or 'I love the florid prose and endless said bookisms of 100 years ago and people still buy the Bronte sister's stuff' is fine if you just want to write it, but not if you expect to sell it in the current marketplace.