That "remnant" is much larger than the handset division they sold. 114k employees after acquiring Alcatel-Lucent. Seems to be doing well. (I just learned that Bell Labs is now "Nokia Bell Labs".)
One can always say that there are other mistakes Nokia did. Like, why did it not acquire Cisco back around 1990 when it could have done it? But then again, many other Nokia acquisitions did not work so well so very likely Nokia Cisco wouldn't have been anything like Cisco is now.
Nokia came to telecom infra because before that it was a cable manufacturer, and it came to cable manufacturing due to being a rubber product manufacturer, and before that it was also wood industry manufacturer. You can still by "Nokian" brand boots and shoes in Europe, although it's a separate company, and you can also buy "Nokian" brand car and bicycle tyres, though that is another separate company.
(I worked in Nokia's infra side for 20 years and 1 day.)