Do I really want a mix of programmers on my project?
That's a very sweeping blanket statement and you're putting a lot of your beliefs on to other people. The smoothest projects I've worked on were always full of nothing but senior developers. But even though that is just my personal experience I know full well many projects benefit from Junior developers. Sometimes fresh perspective and energy are what you want to solve a problem instead of experience and expertise.
For a short time only senior is better. However eventually someone will leave and it takes time to get the new person up to speed. Your seniors will retire and you need juniors trained up and ready to take over. If you are only a short term next quarter thinker you are right, but if you think long term some juniors that you bring up to senior are the better value. (note that this also requires retention of existing people, if your culture is change the team every project there is no value in this - I'm against such cultures, but it is very common in the real world to not value experience on your project)
Of course there is value in bringing in external experts for a short time as well. You don't want to be isolated and not learn from the rest of the world, and of course sometimes you have money for a large project and sometimes you don't and want to fall back the the minimum staff needed to keep institutional knowledge alive.
Last, ignoring everything else, you have a moral obligation to society to build a better world. That includes training the next generation. (and this helps you - when you are retired they will be the senior engineers building the tools to keep you alive)
A lot of these are good points and need to be taken into account. But not every team persists, or even intends to persist. Not every developer is suitable for training others.
I've been on Plenty of teams that formed and disbanded in 3 months, we had a goal of building a prototype and turned it over to the customer. Just isn't a great place for new devs. Alternatively, I worked at slow and steady insurance companies that had all the appropriate processes for risk aversion in place and were great places to teach new devs. They let them stretch their wings knowing that they would be protected by good CI and good unit tests and other defensive practices at an organizational level.
Not every team needs every sort of skill or skill level. It's a judgment called each team needs to make, and some teams will decide wrong.
That's a very sweeping blanket statement and you're putting a lot of your beliefs on to other people. The smoothest projects I've worked on were always full of nothing but senior developers. But even though that is just my personal experience I know full well many projects benefit from Junior developers. Sometimes fresh perspective and energy are what you want to solve a problem instead of experience and expertise.