1) it’s a waste of resources for seniors to be doing the work that the juniors and 2s can do. Hunting down infrequent or low priority bugs, fixing small layout issues, etc. they’re perfect for someone paid less and growing and learning the codebase.
There is *always* plenty of junior-ready work and the day a week of work to schedule, prep and help those juniors to do it pays dividends.
2) a Junior leaving because you won’t pay them, have a toxic culture or won’t give them a promotion when it’s time speaks more about a broken company culture and one of a style that’s rampant in the tech industry and business at large than it does about loyalty and willingness for the employee to stay at the company.
Good leadership and skilled organizers can easily solve the problems you’ve listed; and, even better, create a culture of longevity for all the employees at a company, not just the juniors.
Speaking as someone that has worked for companies that give a shit about their workers and who helped raise me up from a low SDE.