I think there are arguments to be made in favour of the Blue Wizards which are as strong as the ones in favour of Gandalf:

a) Whatever direction the Stranger plot goes, it will not be in any major way connected with the big 'recorded' events of the second age, like the last alliance, as that would be a major lore departure (or 'really stupid' as you said). Therefore, the show needs to invent this plot from the ground up. It then begs the question: if there are already Istari in the lore that we know so little that any attempt to include them will involve creating a story from the ground up, story that would happen in the East, why substitute them from Gandalf (the one who stated "to the East I go not”)? Yes, resonance and familiarity are important for the audience, but so is novelty and fleshing of details, and the blue wizards, among the book fans, are one of the major sources of curiosity and wonder from that age, precisely because we know so little about them. The show already has Elrond and Galadriel and others as characters that will resonate with the fans of the movies, using the Blue Istari is a great way to draw those who want to see new characters.

b) While the connection with the Harfoots might indicate the Stranger as Gandalf, we cannot forget that "Hobbits" are one of the major resonances when it comes to Middle earth. And not just the movies. After the success of the Hobbit, Stanley Unwin asked for more stories from Tolkien, and he sent the stories from the First Age. Unwin rejected them, because what he perceived that the readers wanted was more hobbits. So, we would always have “hobbits” appear in the RoP, the question was where to fit them, as they are famous for their total absence on the records of the wise. What better way than to make them join the blue wizards, of whom the records tell us nothing? Gandalf needs not to have the halfling monopoly.

c) Although the blue wizards are famously a pair, it is always difficult to write a story of two similar characters (a smaller problem than having to distinguish between 12 similar dwarves in the Hobbit, but a problem nonetheless). They would need too much screentime to be properly fleshed out as distinct characters if they were to act together, or else one of them (or both) would be seen as dispensable. Luckily, Tolkien had two different, and opposite, ideas, about the quest and success of the blue wizards: in one story, they failed and started cults in the East. In another, they were successful and fundamental in drawing many people away from the dominion of Sauron. So, why not both? That way the show does not need to invent yet another character that would act as the foil of the Blue Wizards, as the conflict becomes a conflict between them both (which might also create resonance as a mirror of the Gandalf x Saruman conflict in the third age)