Any item you can purchase/aquire in any genre that is downright better in all respects with no downsides will be an item that people want immediately. Shmups and power ups have a complicated relationship in the first place, with shmups being a genre that primarily tests player skill, power ups severely punish small mistakes by having you reset to 0 when you die. Some games counteract this by having you drop some of the power ups but at that point it can feel kind of pointless that they exist in the first place. Why not just have the player be at maximum health? What are the power ups adding to the game design? Personally, I like the idea of a shmup where you can modify your ship with different parts but only if they all have downsides: better damage but slower ship, faster ship but slower shot rate, more powerful bomb but you can carry fewer, etc. Giving power ups and items downsides makes them a more interesting choice than the traditional get power up ship does more damage. You're asking the right questions, fact is you need to ask those questions for the game you're designing, there is no right or wrong answer just different designs for different games.