The concept probably won't have any suitable metrics to be used as a gauge so I don't think there is any absolute answer to this.

With the growth of Bitcoin, you're going to inevitably eliminate the smaller scale miners who can't afford to keep their costs down. It would result in having miners probably being centralized around certain regions and farms. Mining would probably never be any more decentralized.

As for the full node count, I suspect that some nodes actively block Bitnodes crawlers as they publicly list their IPs and thus resulting in a much lower node count. Running a full node is obviously not so viable for most people; even in 2021 360GB+ is fairly significant if you consider that some laptops and/or desktops only ship with 1TB of storage. That, combined with the very few advantages over SPV (which I assume most actually wouldn't consider), will probably reduce the full node count over time. I can't see the point of most people running Bitcoin Core when using Electrum or Wasabi would suffice for their use case while wasting no time to synchronize whenever they're using it.