I get my phone tomorrow and I'm fighting the urge to send it back because of this thread lol. I definitely am not a hardcore phone user; Twitter, Facebook, reddit etc. are all I really use with some light gaming at times. I feel like the phone would be more than fine for that kind of usage. That said, I very much like to "future-proof" and considering my Nexus 5 lasted me five years I want this to do the same. People are throwing around terms like "underpowered" and "substantial downgrade" compared to other phones, and I can't tell if this is hyperbole or not lol.
The Pixel 3a is objectively underpowered compared to modern flagships. It's fine as a midrange phone, and I think that even significantly less powerful phones like the Nokia 6.1 or Moto X4 are more than enough for the vast majority of phone users. When comparing it to the Samsung Galaxy S8/S9/S10/etc., though, the numbers are night and day.
If you're into heavy multitasking, demanding games, or running desktop-style processes on the device, you'll definitely feel the pinch. But if you're using it for streaming medians playback, regular web browsing, social media, simple gaming, messaging, etc., it's plenty fast.
Wide scale 5G is coming within the next two to three years. No current phone is going to be future-proof for five years.
I'm happy with my Pixel 3a (apart from the color shift when not viewing it dead-on). I do switch phones with some frequency, admittedly, so I'm not overly concerned about how it will hold up in 3+ years. If it lasts me through 2021, that's fine by me.