If Freenode disappears, people will join Debian over on OFTC. I'm not sure if Mozilla and GNU/FSF use Freenode or run their own infrastructure but if the former, then they may set up their own network(s).
Other IRC networks than Freenode and OFTC exist too.
As far as security goes, SSL is an option for client<->server connections. I hope that at this point it is default for server<->server connections. What else are you expecting for security?
SSL is not required and it's certainly not default in most clients.
IRC won't survive the loss of Freenode. With Quakenet dying off (its userbase went from ~65k in 2013 to 23k today), Freenode is the last central bastion that keeps the protocol in people's minds.
FOSS projects today are migrating off IRC, onto Slack and Discord instead. This is why you have an article here talking about it and it's not the first one. If Freenode disappeared, this would accelerate massively. The projects won't bother going to another network because the topic of "Where do we go now?" is going to be asked and in most cases is going to be answered with "Well, we wanted to move to Slack/Discord anyway...".
There will be a few projects left on OFTC, certainly. But they'll give up and go elsewhere eventually as well when they'll see their userbase shrinking 5-10x due to other projects migrating elsewhere and taking their users with them. They'll probably end up on Matrix.
Other IRC networks than Freenode and OFTC exist too.
As far as security goes, SSL is an option for client<->server connections. I hope that at this point it is default for server<->server connections. What else are you expecting for security?