They could hardcode the servers IP, but then they can't ever move their infrastructure (horrible idea). So they do the reasonable thing and put the name in DNS. Where do you imagine the lookup for this .local name being directed, and how will it end up at TP-Link's current management server?
Either way, the domain only makes sense locally. This is also why the domains still work even though they no longer own them. Therefore, if there were a local TLD, this would be the proper use-case for it.
people would type login tplink into google/bing
from a usability standpoint this solution wouldnt work.
people would type in login .tplink .com
https://www.google.com/search?q=login+.tplink+.com then 301
people would type login tplink into google/bing
https://www.google.com/search?q=login+tplink then 301
This serious problem you describe, of cavemen and preschoolers who don't know about URLs, has been solved now for a long time. 301 is part of that solution. The tp-link.com site is perfectly capable of examining Referer and using that information to help tplink users.
However, since we've already established upthread that the router controls everything about the online experience, it would be no problem redirect the first navigation through an unconfigured router to a "wizard" page.
there is no reason to insult the people in their 20's 30's 40's 50's 60's 70's or 80's who are confused about the difference between a search bar and address bar (especially considering the major browsers merge them)
the unified bar has led to a sharp decline in people being able or caring to type correctly configured urls.
I also do not know anything about the configuration of these devices, i just assumed it is just a convenient way to access the local web interface for configuration of the device. If there really is a centralized service on the internet needed to configure the device it would be somewhat problematic to set these devices up or not?
$ nslookup tplinklogin.net
Server: 192.168.1.1
Address: 192.168.1.1#53
Non-authoritative answer:
Name: tplinklogin.net
Address: 192.168.1.1
---
An external request:
$ nslookup tplinklogin.net
Server: 2001:4860:4860::8844
Address: 2001:4860:4860::8844#53
Non-authoritative answer:
Name: tplinklogin.net
Address: 103.224.212.249