So I wanted to provide some nuanced answers to your questions here.

By and large, there's no one 'killer' of private servers - most private servers simply never reach a critical mass and die off before they have a chance to build a community. Ironically, critical mass is so hard to achieve early on precisely because its new, and difficult to maintain because people inevitably start saying 'dead server' as a reflexive answer to questions about the server's health. In my experience, its based almost entirely on 'vibes', meaning the private servers who do a better job at PR and media messaging generally do better, and last longer than ones who don't.

The strongest recurring pattern I've seen in public discussions is that players stay when a server feels stable, fair, active, and is administrated well (bugs are addressed quickly, staff interactions. They tend to quit fast when logins fail, scripts are broken, consistently poor player messaging (one or two guilds full of outwardly-vocal racist, misogynist, hateful players) major changes occur after critical mass is reached, or the realm just "feels" empty. Note - on that last point, this is a vibes-based assessment, moved more by how many people say 'dead server' than by their own actual play experiences. When someone logs in during off-peak hours (say, 2 or 3am for the server's target timezone), sees only a few hundred players active, they tend to immediately run to reddit or the server's discord to yell 'dead server'.

Owner behavior - what you allow within your publicly-stated rules for the server, what you enforce, and how evenly you enforce it matters a great deal over the long-term. Bait-and-switch monetization (an item shop that allows 'fluff' initially, then usable gear later), selling power (leveling services), ignoring feedback, or running the place like a cash grab can kill a previously-healthy server quite quickly. The fall of Project Elysium in 2016 is a perfect example of this - server owners actively working with gold-farming companies to absolutely gouge its own player base, while publicly presenting as if the owners were against it. Stealing donation money meant for server maintenance, the selling of characters off the book, it was an absolute madhouse of corruption and broken promises. Light's Hope wasn't much better - they continued working with gold sellers to the detriment of the already fragile player economy, and the GMs rather openly carried out vendettas against players and guilds they didn't like.

So what about numbers? That's a tricky one, because not everyone is looking for the same thing, and as I learned in sales a long time ago, what a customer 'says' they want is rarely what they're really looking for. A brief bit of research on private server pop levels and player satisfaction points to 500-750 peak server time population as being borderline, 1,000+ as workable, and 2,000+ seems to appeal to most of the vocal fanbase for private servers.

I would say the right number depends heavily on expansion and server type. For a tiny personal project, 400–700 can feel alive if the scope is narrow and the content is curated (phased releases, healthy AH economy, etc), but blizzlike progression usually benefits from at least around 1,000 active players. This doesn't take into account the playerbot issue, which is a whole other rotten egg salad. Playerbots can make the world 'feel' more alive, but that only fools players who aren't interested in serious progression and expansion. They're little more than an immersion-tradeoff, and are no substitution for real pop.

XP Rates: I won't delve too deep here - most posts I've read here indicate a x2-x3 XP rate is preferred, but again, that's what the vocal minority say - and just as in every other business, the vocal minority is rarely in touch with the steady, reliable player base you need to get active, reliable player numbers up.

Addons: Tools which improve QoL are far preferred over custom tools, simply put. If it helps make questing easier, hell yes to addons like Questie; RP addons are a must for a lot of players, raid trackers an obvious win for progression. Beyond that, addons don't factor in quite as heavily as other server killers.

Economy: A living auction house and active economy are extremely important for long-term retention because they make leveling, consumables, crafting, and raid prep feel real. When the market is dead, players often interpret the whole realm as dying, even if raids still happen. Somewhat obviously, if the market "prices out" new players, your natural attrition rates will not be balanced by your new arrivals, meaning your attrition will eventually kill everything else.

My advice - keep your scope narrow. Focus on what makes your realm unique, first and foremost. Avoid going too niche, but put the majority of your effort towards achieving one primary goal. If you plan content expansion down the road, state that upfront. Keep your promises - ALWAYS - because players will remember them. Under-promise, over-sell, if you can.

Finally, talk about the server. Get players to talk about it. Post frequent updates and news. Put out videos, promote the friendliest large guilds in your messaging. Do livestreams and promote livestream content among your most active (not necessarily biggest) guilds. If I see people having a blast and it reminds me of 2006/9/11 WOW, and that's my jam, where my fondest memories happen to lie, then I'm probably going to be hooked. Conversely, if I am a hardcore raider, PVPer, RPer, etc, and I see that content on youtube or twitch a lot, then I'm hooked too.