Joined October 2015, currently level 40.
When I hit level 50 I doubt I’ll ride any differently. 
Personally, I’m not motivated by Zwift levels, especially since the Drop Shop. Unlike an MMORPG, for example, they don’t confer any additional power or abilities. Unlike most other games, they’re not visible to anyone else except in a small set of circumstances.
I don’t really understand how Zwift view levels these days. It’s three years since they last raised the level cap. Is this because they have abandoned levels as a progression and reward mechanism? The Drop Shop was launched a mere two months after levels 26-50. It’s possible Zwift deliberately deprecated the level mechanism with its curated rewards in favour of the system that provides greater rider agency.
That doesn’t seem the case from the Eric Min quotation cited earlier in this thread (Level 50+ update? - #183 by MikeBikes66), but it’s possible the proportion of Zwifters who have reached level 50 is small enough that it’s not a priority in Zwift’s eyes.
I’ve no idea, and I’m not about to start tagging Zwift staff demanding insight, but it’d be welcome to get something. Even if it’s “50 is the maximum level and we’ve no plans to increase it.”
As for Rouvy being an alternative, wouldn’t that suffer from the same problem? Once you’ve reached “Legend” in career mode, you’ve beaten the game. I suppose you have the progression from Starter to Legend to play through, but ultimately there’s a ceiling.