Given .NET's annual release cadence, 7-8 years of LTS would mean 5 concurrent versions: 1 STS release and 4 LTS releases. This is still a lot of old versions to keep updating, of course.
Anyway, the critique was less about the exact length of LTS, and more about the lack of significant difference between LTS and STS, i.e. why should I choose LTS in the first place. With STS, you have 2 years of support, and 1 year to upgrade. With LTS, you have 3 years of support, and 1 year to upgrade.
I think Microsoft regretted those long (10 years) support guarantees they had for .net framework. In the end, it probably slows you down in terms of innovating