How intense can I make my (PC) game?

I want to make a game like DMC4 where you can do stuff like this:

On consoles gamers complain about games only being 30 fps and think drops below 30 fps are unacceptable.

Ideally I would like to hard code my game for 60 fps.

But what do I do when the fps goes too low?

Do I just ignore it and consider that undefined behavior?

Or should I put tons of effort on making sure it behaves exactly the same no matter what?

For example: if the game is designed for 60 fps, but ends up running at 15 fps for some reason, then should the game run in slowmo or should I allow things to happen “logically” that you won’t even see on the screen (as the “sample-rate” is too low).

What do you guys do?

What if the game gets unbeatable if the fps is below some value? Do I just put a message onscreen saying get a better PC then?

If you do things right, framerate drops won’t be a problem for this.
Games like this does not abuse the physics system. I’m working on a game with lot’s of physical manipulation and interactions still fps dropping won’t be a problem.
The animations makes you think that there’s a LOT of collisions going on, but it’s not. It’s just a matter of well designed integration between actions fired / hit detection / animation execution.
Nothing there is abusing physics, not even sword hits. Those hits are plain mimic animations, see animation notifies:

A simple role is, if you’ve relying too much on the physics engine for everything, you’re doing something(s) wrong.

But, I’m the kind of person that likes to find ways to break the stuff that I do and often wonder, “what if”…

Like what if the antivirus program starts DOS-ing/hogging my PC

Well, choose your minimum spec hardware and develop for it. Buy some lower or mid range hardware that you want to be your minimum system requirements at your minimum desired framerate (whether that be 30fps or 60fps) and squeeze as much out of that computer as you can performance wise, with model detail, shader complexity, and physics objects. Anything below that minimum recommended specification the user should understand they’re in uncharted territory and poor performance is their fault for not adhering to your advice as the developer. That said, you should be doing profiling to optimize the slower aspects of your game, determine whether you are CPU or GPU bound, and thus refining your system requirements as you get to launch. But for instance, if the vast majority of your game runs at say a reasonable 45fps on your min-spec rig but one area drops to 15fps, that’s on you to optimize it. You shouldn’t have that degree of variance.

In regards to other programs… that ain’t your problem. Tell the user to temporarily disable their antivirus if you think it’ll cause problems. Most people with lower end computers already understand they have to pretty much shut down every other program when gaming.