So I started late July last year, on a server that had been up for about 2-3 days, give or take- so I've been playing none stop since then, 6 months ish.

I'm going to answer your questions in order, some have already been given an answer but I'll add my opinion on what you should do as a returning player, but this is dependant on your knowledge about the game, not how much you think you know either, actually know.

Firstly, the developers. I highly doubt they even know of our wants here, Oasis "say" they pass on information, but I've played a few games with publishers that just get the product from another region translated, the common situation is this- They get the game "As is", translated, there will never be any huge change, or balance edits because most of it has been out & done already in other regions for more than a year, so we're bascally getting a product in increments like a Pizza, and we get given each slice as "new" content when it's given, now when it's given is most likely decided by Oasis or when it's translated.

A new servers longevity I guess depends, on a lot of factors and those factors can range, but people blame how hard someone pays to win or the most likely reason, things don't last forever & there will always be a drop off of players.
I'd actually speak quite highly for the server I was on, it was actively holding 3-4 guilds for GNW, that were actually close in competing for the first spot for a few months, that activity declined heavily through the months with even leaders leaving, but when one guild died, it would merge with the next least active- so there was always a fair balance- I think this was more due to the players though, we had a good selection of decent minded players.
Near the end, toward our server merge we had only 2 active guilds which merged when the server merged. The merge breathed new life into the server & a lot expected it to be for the worse, but it wasn't actually that bad.
Since starting I've joined a few servers, early testing showed servers being very active but most servers get emptier every time I try a new one- one of the most recent LA ones for example had like a single guild with 30 players on it for example, it was actually as poorly populated as mine was right before a merge.
They seem to be getting wose- but I haven't tried every single new server, so it could be a Region by Region thing, or it was just random.

My experience with Oasis is, meh. I did a lot of research on them & the game during my play, there's a lot of bad history there & if you dig you will uncover a body- It's really not good & I honestly don't think they need to be anything more because they're getting a product they put no work into that is making a lot of money, they're disconnected from their player base- but they don't need to be connected because at the end of the day it's a P2W game and as long as the guys at the top are happy & bringing them the money, legit nothing else matters, because the bottom line is the game can survive on whales alone, it was built too & most easy to create pay2win games are especially with the added caveat being that it's Naruto- it's too big to fail you could say.

Bottom line: Do you think I should return to this game on a new server?
Did you enjoy it last time you played? Knowing what to expect from your previous time, if yes- Sure, I enjoy it. I've spend a few hundred on the game so by that definition I'm not free2play, but even at a few hundred I feel like one, there are players that spend thousands, if you can get past this & just enjoy the game for what it is, I would recommend doing this, play on a server learn as much as you can unto level 80- or do that research beforehand then start again.

I made a post on reddit about this that I wish I wasn't lazy enough to find for you but I'll try to remember some things.

- Try to keep your skipping on dailys & events to a minimum, this one is probably huge. They're the only way to increase your power gradually.

- Save everything for rebates, Refines, Charms, threads & anything else I've missed- this is including seal scrolls. If you have the will power to do this, you will in the long term be far better off. The reason for this is, most of the time you wont win events unless you pay2win, so those 10-16 Advanced Refines you get for Sage World every time are off the table, so you want to maximize in other places. If you p2w, take the reward and still only spend on rebates.

- At around level 70, slow your roll- for example, don't do story missions unless you really need them, or just don't do them at all & naturally progress in level, you will gain more from it in the long run competing in lower level events, while your power is still increasing.

- If you decide to spend, spend on power over everything else & only spend on Ninja if you have done the research and figured out what team you want to aim for, choose something realistic don't go for Naruto 9 tails Chakra, or the Edo Deidara, Earth Grudge team- they're incredibly expensive.
A common way of doing things is to get a Barrier ninja & create a team around that Barrier Ninja, or go for a GNW treasure mist team. Or pick up something from a Lucky Star event & build something around him.
With higher initiative & power you can beat mostly anything- because that's the majority of what wins you a fight.

- Lastly, coupon spending. Save them for things you want, power or a Ninja- don't spend on things you don't understand & check a youtube video for more information.
Math will help you here, try & figure out what is worth spending on & what isn't. Generally the monthly Lucky Board is great to spend on for power early, because it gives quite a lot, but there are events all over the place that are great for coupon spending & some that fall off the more power you gains, like the Lucky Board.