I've tried to make it through most of the constructive posts in this thread. I'm also an AE/TC lover and have had several iterations, currently an 802.11ac Airport Time Capsule. I'm definitely on the "maybe buy a backup or two" boat, but I also don't like the idea of having deprecated hardware around.
I'd be interested in thoughts from others.
I had the Synology 2600, highly touted as it is, but went back to the TC for simplicity's sake (and that it didn't accomplish anything for me that the TC didn't, but that's my use case). I do utilize Time Machine with an encrypted backup to the Time Capsule via 802.11ac as one of my backups. I'm aware that TM can be done via (some) non-Apple hardware.
I've seen the posts in this thread about the Velop and the Eero, though I've never tried mesh networking devices and am unsure if they'd be of measurable benefit to me.
I have a 100 Mbps cable connection in a two-story house. I occasionally stream 4K, frequently stream HD otherwise, and occasionally have multiple devices streaming with no hiccups to this point. In-network, I can see connection speeds anywhere from capped at ~1.3 Gbps right adjacent the TC to ~400-500 Mbps for 5 GHz -ac connected devices in adjacent rooms with fall-off down to the 100-200 Mbps range for more distant devices. I have a media setup far from the TC which is often down to that 100-200 Mbps range, but I've not noticed any real-life issues there thus far.
All that said -- I'm torn between (a) liking the latest and greatest in my "toys" as tech is a hobby of mine, but (b) being a big proponent of my AirPort TC. I feel like I should probably just go for the Eero or Velop or other such modern setup, but are the benefits really there given the above? I'd be losing out on familiarity and ability to use Time Machine natively to the TC, but other than that, I don't know.