What happens when your sysadmins say "Maintaining IRC is a waste of time and gets us 1% of the features Slack does, let's just get Slack?" (At my last company, it was the sysadmin team, including myself, who decided to switch to Slack and killed our IRC server.)
Yes at that scale it may be useful to roll out a custom system. However I'd count into account that large customers probably have different pricing and that maintaining custom systems is always bothersome because you are always the largest customer.
Get 2 sysadmins. 2 because of redundancy. Pay them to maintain your service.
"well funded sales team and corporate support". F that. Trust your employees.