It's better than nothing, but it's a major downgrade. Politics is even more one-sided and harder to avoid; the entire website is an echo chamber. Many communities lack population, and Lemmy's sorting algorithms suck.
You only ever see posts about people who are happy with the switch, but I have a hard time believing most people actually feel that way. The ones who are disappointed probably just don't get upvoted.
To keep old reddit alive they need to also spend developer time supporting some new features and not breaking legacy code. It's more than just server costs.
Speculation, but it's an inevitability. It costs money to support old.reddit, which means Reddit will certainly kill it on the day that they'll think the backlash will be small enough.
A tale as old as computers. Here's a very good talk about it: https://youtu.be/kZRE7HIO3vk