Why would you think Nintendo is going to improve the online, especially if Super Mario Party has **** online?
It's only logical to assume that by paying for the service, it would get better than what has historically been a difficult medium. Nintendo tried their hardest to please everyone, but between lag issues, spammy players, etc, it just ended up alienating a large portion of the smash community.
For Glory was probably the best option they could have come up with when they devised online for WiiU/3DS. While still free, it at least made it so more serious players could enjoy a version of play that was intended to connect like minded players from various regions without the use of matchmaking websites, etc. It was definitely an improvement over Brawl's online experience, and it didn't cost anything extra!
That said, there was obviously room for improvement. Enter Ultimate: here we have gone a step further. Instead of "For Glory" which was a static match type, you can now enjoy endless combinations of rulesets. I anticipate once the BR is able to issue a statement on ruleset, that many players will opt to try online since you'll be able to specify stocks, stages, etc.
That's already an improvement, technically.
So really, your question focuses on the elephant in the room. LAAAAG.
I hate to say it, Lag issues will probably always exist. It's the nature of the internet. No matter how much you spend, and no matter the setup you may have, the service you may purchase, factors beyond consumer control do and will play a role in whether or not your connection and experience are "just as good as offline." Even as I'm typing this, due to the ice storms in our region, there is a bit of lag lol. We pay a good deal of money each month for our internet service. But it's par for the course. We don't expect it to be "better" just cause we're paying money. What we DO expect, is for the service to use our monthly payments towards research and development in finding better ways to provide service, which we then will benefit from once they're implemented.
Seeing as how Nintendo has never really charged for their online portions of games aside from paid DLC, I believe that a subscription service would play out in much the same way. It's not all profit in other words. This money will most definitely go towards maximizing the online experience beyond what Sakurai has already done in terms of revamping the mode on the software side.
Hope that answers your question!