I think we'll be able to find these for quite some time (you can still find wrt54gs on amazon...), but the writing is on the wall. Unfortunately it doesn't look like there's a ton of agreement on what would be the successor:
"The WDR3600/4300/4900 seem to be EOL here too (the Netherlands), which
is pretty frustrating, as I wanted to use the WDR3600 for some purposes."
It looks like the next iteration might be the tp-link archer c2 which is an AC device, but is using ath10k drivers for 5ghz and might not yet be completely supported in openwrt. Not something that I think we really need to worry much about :)