Fair question. In hypothetical situations I tend to think more towards the darker consequences/outcomes that get pushed aside or forgotten for an article/story/daydream so here's how I see it:

Increasing productivity means more resources are needed though. During previous expansions, this wasn't much of an issue however now it could be the genesis for a new world war so 'the powers that be' whether our known governments and militaries or some 'secret shadow government' both would likely be hesitant to unleash really cheap and bountiful energy.

It would be great for powering our homes sure, but you'd suddenly get every major automotive manufacturer seriously looking at EVs because who wants to pay 2.85 a gallon of gas when the equivalent in electricity is ten cents? So great, EVs for everyone... but where do you get the precious metals and the lithium and other rare earths from when you suddenly increase the demand one or more orders of magnitude?

Then electronics developers stop worrying about being as energy efficient as possible, so you start generating a ton more waste heat and you've got people being more liberal with their air con or heating at their homes which goes through refrigerants faster and goes through heating elements faster.

And with cheaper energy people start driving more for road trips, even with EVs, you start to see more and more rubber going into the environment via the tires. And with that cheaper energy, fuel prices go down and more people start flying places and further disrupting resource allocation/logistics globally.

And with that cheaper electricity, those autonomous EV delivery vehicles become the norm and you're always 30 minutes away from Uber eats or Amazon Prime Now and "oh man, I dropped my toothbrush in the toilet, well I'll order another and it should be here while I shower" and that's 20 miles more of rubber deposited onto the roads, another cardboard box, another insulated container because once you got out of the shower you decided you wanted some ice cream and placed a second order which is another 20 miles worth of rubber and an insulated container into your trash...

Dirt cheap, clean, unlimited energy would be the greatest thing ever to happen to humanity and quite possibly the worst thing to ever happen to humanity as it would be a genesis for even greater amounts of disposable goods.

The impact it would have on mining alone could be incredibly damaging to the planet. A lot of mining equipment is still fossil fuel powered which severely restricts how deep/far/etc you can go and at what concentrations mining a given location stops being worth it. When you effectively remove the energy costs, you can mine much lower concentrations and go much deeper and even mine far more remote (and pristine) areas because you could fly in a reactor in cargo containers and assemble it on site.

I'm all for fusion, for space exploration alone it would be monumental. We could send probes to nearby stars and have return missions within a decade or two if you spend the bulk of the first half accelerating with ion propulsion, then flip and start reversing thrust, get some data in system, slingshot around the local star and start accelerating back...

But I see a lot of potential bad that would come from affordable and scalable fusion (or magical zero point energy for the sake of argument) as we've become a "use my phone for 18 months and throw it away for a new one" and "let's fly to see the pandas/tigers/whales/elephants/penguins next week for our third international exotic animal trip this year" society.

We're generally pretty poor stewards of our planet as a species and while unlimited clean energy would certainly get us off fossil fuels, it wouldn't stop the many other ways we harm our natural and man made environment.