You can read the relatively weak sales and unpopularity of Simpson's House in 2 ways. On the supply side, they haven't had a lot of sales. This is actually pretty good news for investors, you'll have less competition compared to hugely popular lines like the modulars. On the other hand, it also indicates suppressed demand, at least at this stage while selling to the Lego masses.
A key question then becomes, what will demand do in the long term, after EOL? That's a judgement call.
My opinion, nothing more, is that this set may gain in popularity after retirement, especially with KM and possibly additional sets in the theme. Also, this is an AFOL set, and many AFOL don't know they want Lego yet; we get a steady stream of new adults (often new parents buying for kids) that "find" Lego each year, then proceed to build collections for themselves. A Simpsons theme is well-aligned with AFOL collectors (as previously mentioned in this thread). This gives retired sets new life, and explains why the collectable themes raise the value of early entries (Modulars, Star Wars UCS sets, etc.)
Now here's the trick: how do you combine these bits of data to find the relative desirability for investment? Stated differently, how do you find sets where the demand/supply ratio may move upwards after EOL? Low initial supply as evidenced by weak sales + delayed demand (speculating here) is a pretty good flag. Given that, I actually think SH is a decent investment, even though sales have been lackluster.