This question is somewhat confusing; a lot of the language is vague and unexplained. From what I can understand, you're suggesting that single-type Pokemon are at an inherent disadvantage, and there should be some measure to counteract it or they should simply get second types? In particular, I'd appreciate an explanation of what you mean by having a "diluted type" or "half and half being full and all", but regardless I'm going to answer the question since I find this topic interesting.
If I am correct in my assumption that you're asking why single-type Pokemon don't get an extra advantage, know that pure-type Pokemon still have tons of potential to be effective. Having a single type does not immediately make a Pokemon a bad option, because that would be ignorant of all the other possibilities and factors to consider in Pokemon. While there is no direct compensation for the lack of a secondary STAB attack on single-type Pokemon, they are at no defensive disadvantage. All that happens is they can't have double weaknesses or resistances; there is still a wealth of good "pure" defensive types, like Water, Fairy and Steel to name a few, and perhaps to a lesser extent Dragon.
In addition, all the worst defensive types are dual-types because having two types immediately means there's potential that the types won't counteract each other's flaws and will synergise badly overall. For example, Ice and Grass or Psychic and Grass are both poor combinations because together they're weak to so many different types, and don't resist enough others or pack the STAB coverage to speak for it. There's such a thing as a redundant secondary type too; offensively, Pokemon like Dragonite and Salamence don't value their Flying type much, because it offers them no coverage of the types Dragon cannot break through. It could easily be argued that the second typing is simply a liability, as it gives them a suboptimal double Ice weakness and dreaded Stealth Rock weakness. The free switch to Ground attacks is somewhat helpful, but Flying is of little competitive use to these Pokemon otherwise and simply gives the opponent more opportunity to exploit weaknesses. You could also argue that the best types will all statistically be dual types, but this doesn't mean single-type Pokemon can't be effective as well.
Also consider that type is only one of many different factors to consider in a Pokemon; it is very hard to give a label to single type Pokemon when that covers such a broad and diverse amount of Pokemon with different factors, variables and playstyles that might affect how they play. For example, a Pokemon like Aromatisse would actually be disadvantaged if I slapped Ice type onto it along with Fairy, because Ice is a poor defensive typing and Aromatisse has little use for the type's offensive capability being a cleric/support Pokemon by design. It would have the Stealth Rock weakness, and would miss its Fightning resistance in particular.
As a long time competitive player myself, not once have I ever thought not to use a Pokemon purely for having one type; I instead consider what overall value they can provide to the team I am building and what metagame threats it would support against. Perhaps the Pokemon could be improved and made more favourable if it had a secondary type, but remember that the exact reverse can be argued for Pokemon like Aromatisse. This subject is inevitably subjective, but I think's another reason why it's unhelpful to give labels to large groups like "bad decision" when there are so many other ways to look at a Pokemon.
Defensively at least, it might help to simply see dual types not as a combination of type types, but as unique types with different sets of weaknesses and resistances each. When you think of it that way, it removes any pre-existing judgements and helps you look at it more literally. Compare it to some of the single-typed Pokemon: statistically, you'll probably notice that "pure" typed Pokemon have more neutral relationships with types and have no double weaknesses or resistances. Is that really a bad thing, or is it just different? Again, the offensive disadvantage is unarguable, but you could also see it that those Pokemon were designed with that in mind and some of the choices made with their stats or abilities may have been made to accomodate this offensive disadvantage.
Basically, the point I'm trying to make is that single-typed Pokemon aren't inherently bad as you seem to suggest, because there are so many other factors to consider and because this is highly situational. A dual type could be good, or it could be bad, and again why labelling large diverse groups like that doesn't really get you anywhere. There's still value in your point though: maybe single-type Pokemon should be supported some way, or maybe they should just continue being designed and supported individually so that no blanket change advantages or disadvantages any specific Pokemon differently?
Sorry this got so long, I hope my insight was helpful in some way.
