If we're speaking on a purely scientific basis nothing truly needs anything. It's like when people say science says that we need to do X. No, science does not say we need to do anything. What scientific studies say is that in order to have a specific outcome we need to do certain things.
Science doesn't say that you need to breathe, that is an incomplete thought. Science does not have any kind of moral imperative attached, it is just a method of analysis, a way of looking at a situation.
However, science does say that you need to breathe IF you want to survive.
Based on studies of incomplete metamorphosis it also shows that a kakuna, metapod, cascoon, silcoon, or any other Pokémon with a cocoon stage WILL undergo immense physical discomfort and strain if not permitted to evolve, and will also eventually die.
I am going to use a wurmple as my example, because those are what we have at my museum. The specific form of metamorphosis undergone by bug Pokémon with cocoons (or pupa as they can be called) involves a step known as pupation.
The life stages are larval (wurmple) then pupa (silcoon or cascoon) and then adult (beautifly and dustox)
First of all, in the cocoon form (as pupa) they can not eat. So the amount of time they can spend in the form is limited by whatever nutrient reserves they have collected during their larval form. These nutrients are intended to go towards metamorphosis.
Second, many of the larval structures begin to break downs and are disolved, basically melted into individual tissues. These tissues and some of nutrients from those are then used to build the adult structures.
Even with an everstone, this process is still going on, and even if it wasn't, because the pupa is unable to eat it will slowly starve to death. The tissues will eventually be broken down for energy, and because the Pokémon is relatively immobile during that stage in order to conserve its energy, it will take a long time to starve.
Now the Pokédex will tell you that silcoon drink during their pupation, but, while that is technically true in a way, it's not what you think. The membrane of their pupa is what is called semi-permiable, and will allow moisture through in order to keep the Pokémon properly hydrated. It will also unfortunately let them lose moisture if it's too dry.
But this doesn't provide them any kind of nutrients, and even if you were to try to feed them sugar water or some other kind of nutrient fluid through this method it wouldn't work. The pupa allows in water molecules, and has spiracles that allow oxygen to enter and carbon dioxide to exit, but these channels are small enough that larger molecules like glucose (sugar) or any kind of vitamins just won't travel across the membrane.
Now let's ignore the fact that the lifespan of the Pokémon is dramatically reduced because it's starving to death and has no way to evolve into a form where it can feed itself. If pupation is interrupted some things won't happen. For example, the somewhat squishy abdomen of the wurmple may not properly harden into the more durable exoskeleton of the beautifly. Or, the exoskeleton may harden when they're still stuck inside the pupa, and the Pokémon will become malformed and potentially die.
So no, they don't need to evolve, not evolving is a possibility, and some will naturally fail to evolve, be it because of genetic defect, or environmental conditions, or being malnourished before beginning pupation and thus starving before it's completed.
However, if you want the Pokémon to be healthy and to have a decent quality of life you have to allow it to evolve. Being stuck in pupation is a slow death, and, depending on how long they survive, likely a painful one.
And you can't stop pupation either, keeping a Pokémon in its larval form is just as physically harmful to it as failing to complete pupation.
At our museum we raise hundreds of cascoon and silcoon each year to teach children about metamorphosis, and the sad fact of the matter is that, depending on the year, between 20% to 40% of them fail to survive pupation. Occasionally, depending on the circumstances, we may have to euthanize some because of quality of life issues, as we won't be able to release them in the spring. Others end up weak and we know that once they're released they'll likely end up as food for taillow, swellow, and other bird Pokémon.
If a trainer does not want a bedrill, then they should not capture a weedle, or, when the weedle pupates, should find a new home for them. It is just like getting minun or plusle. Even though they don't evolve, they still grow, and they won't always be an adorable tiny little newly-hatched Pokémon. Things grow and change, and nothing can stay a baby forever.
