I think the reason it hasn't "aged" very well is because there really wasn't a video game industry to the standards that it is today. So maybe Nintendo thought back in the day they'd get some people to purchase their system and develop a few games for it and move onto something else. However, this obviously did not occur. Once people found out that they could develop games for the system, more and more ideas came in, and with every iteration of games that came, more and more people crafted their skills in game making. A few years later, with what was learned with the first batch of NES games, Nintendo (as well as other developers) came up with other ideas and ways to design games, hence SMB3, Super Contra, etc.
Today, it seems that there is some thought put into games (of course not by everyone) when it comes to making sure the game can hold up to the test of time, especially since there is a demand for older platform games on the markets (i.e., PSN, eShop)
So to give my two cents on SMB, I agree it hasn't aged well when you compare it to how games age today or even to the likes of SMB3, but at the same time, you can't really categorize it with those games because it is THE game that brought to life and essentially contributed to what the industry is today. So for that, I will always be thankful and hold a special place in my heart for the original SMB on NES. If it wasn't for that game (and platform) garnering in the HUGE amount of consumers that it did back in the 80s, the gaming industry would have died (lest we not forget the video game crash of '83).