Slack's leadership likely think that Slack is already so ingrained in corporate settings that people will go for it by default, without word of mouth from people using a free plan. There is enough word of mouth from people who used it on a previous job. Sort of like Jira.
I started using Slack in 2015, and never in my life used a free Slack account.
I started using Slack in 2015, and never in my life used a free Slack account.