Because I have gone kinda wild with mine recently.
I don’t want to date myself. But just recently, I realized my VCR is obsolete. (What am I going to do with all those tapes, now?)
I just tonight DVR’d all the Simpson episodes I like. And I have a couple of my favorite movies on it too. (I just couldn’t live without at least one A Christmas Carol.)
When will I ever run out of space? As I said, I have a lot already. 
If you need more space, can’t you hook up more space? Networked mass storage, for example.
All of Simpsons and a few movies does not sound like you will be needing petabytes of storage, in any case.
Googling, the X1 DVR I have from Comcast can hold about 150 hours of HD programs. Of course I never let it get that full. I get uncomfortable once more than half the space is used and start deleting the stuff I know I’ll never watch.
Depending on your DVR (or media player or whatever) you certainly may be able to add external storage via USB, or possibly a NAS. I use a Raid-1 NAS and keep a total of 8TB on-line, but this is pretty unusual. Writing to an external drive or NAS is usually the challenge.
although the OP is asking somewhat factual questions, at its heart it seems a request for opinions and experience. So I moved it to IMHO, which is where this sort of thread is most at home.
I have a 2 TB DVR and that’s about double the storage I use.
Mine is just about half full and still has capacity for 158 hrs of SD programmes. I have managed to copy over a lot on to an external hard drive for when I’m travelling - off hand I’d say about thirty movies, complete runs of a dozen or so comedy and TV series, so I shouldn’t get bored.
The TV itself has a USB slot for adding an external hard drive, but I’m not sure if that’s only for playback on the same TV (would be useful if not, the PVR tuner seems to be susceptible to interference from somewhere at times).