I have seen the "we don't want to waste time teaching people. So, hire only senior developers" scenario play out at couple of my previous employers. And, it doesn't end well.
Senior devs have expectations around salary, growth and work. Over time when the company starts running out of steam - funding, growth or even interesting work it gets difficult to keep the senior devs happy.
Once that scenario plays out, companies start scrambling to find interns or junior devs and find out, not many are not interested in working for them because they heard "rumors" about company's hard-ass culture on "not learning on the job".
Slowly, company realize that a team made of only senior devs is too costly for the value they provide. And company starts to cut down, put in hiring freeze telling the senior devs to produce more per person. Some even go to the extent of firing some seniors and getting junior devs and then allowing people to learn on the job.