Great question.
My first response would be that all desires are pleasurable, and that only those desires that bring more pain than pleasure in net total are clearly out of reasonable bounds.
Yes you can live your life in a cave on bread and water and sustain yourself, and since life in the absence of pain is pleasure, and if you succeed in living without pain, then you have reached "pure pleasure" in an abstract general sense. There is no absolute arbiter that says "you are not living pleasurably enough" and you should make another choice and pursue other sorts of pleasures.
But given the way the universe operates, it is possible for most people to obtain much more pleasure than that. Most people will realize what they are passing up, and they will regret not obtaining what they could have obtained with reasonable cost in pain, and so they will of their own accord feel regret and therefore pain and not be content with their minimalist choice. In most circumstances they will also find that they are not immune to the impact of outside pressures which virtually always occur, whether it be disease or criminals or invasion or whatever. And living strictly minimally is generally not going to prepare you for those hazards.
That's not to say that some will not be content with total minimalism, but there's no absolute rule other than that pleasure is desirable and pain is undesirable, and no one has the natural right to say "this is all anyone needs" and enforce that view on anyone else. They can do that under civil law, but that's not the same as saying that they have a natural philosophical moral right to do so.
So I think everyone has to ask themselves that question: "I can get by on a lot less than I have now, and I can feed and drink and live minimally. Should that be good enough for me?"
I don't think Epicurus would say that everyone should live like that, nor did he live like that himself. Look at the property that he accumulated and disbursed in his will. I would argue that there is no evidence that Epicurus or any other Epicurean ever lived such a "minimaliist" lifestyle. Statements that all one needs is water and bread and cheese have in my view all the markings of "philosophical extreme" statements, meant to prove the point, but by which no one actually lived -- because it's not necessary to live that way, and choosing to do so is generally an abdication of the experience of many other pleasures that are possible in life.
No other animal or infant of any species lives that way - pushing away any pleasure above what it needs to actually "survive" - and neither should we.
-----
That's one way of making the argument.
In addition to that, I think Torquatus gives us additional valuable information about what is going on with the natural and necessary distinction. As Torquatus explains, "the principle of classification being that the necessary desires are gratified with little trouble or expense; the natural desires also require but little, since nature's own riches, which suffice to content her, are both easily procured and limited in amount; but for the imaginary desires no bound or limit can be discovered."
I interpret that to mean that what Epicurus was doing was pointing out a way of analyzing desires so that we can predict their consequences and THEN factor those consequences into our choices. Obviously we need a method of predicting how much pain any given set of choices might bring, and this classification makes perfect sense -- the more extravagant the goal, the more likely it is going to cost a lot in pain to pursue it. That's not saying "don't ever pursue it" - it's saying that this is the way to analyze what to expect. Epicurus does a lot of that, as about sex and marriage for instance. He points out the ways to analyze the advantages and disadvantages, but he doesn't say that there's a flat rule of nature against something.
In the end everyone has to make these decisions for themselves realizing that there's no absolue right or wrong answer or supernatural god to reward or punish you for your choice.
But in the end, for my own analysis, it comes down to: In an eternity of time i am only alive for a very short period, and restricting the amount of pleasure I pursue to only what is necessary to keep me alive is about as foolish a thing as one could possibly do.