The cone of uncertainty has a few uses, but is most useful when used in conjunction with some other practices. So let's start with the uses in the circumstances you describe.
Cone of Uncertainty when a Commitment is Made
Obviously, you can't delay a commitment that is already made, that's a simple matter of physics (time). Most organizations I've worked with that work in the fixed scope and deadline model who are successful do two things. First, they pad their estimates severely.They are always promising the tail end of the cone of uncertainty. Secondly, they retain some control over the effort they can apply, adding teams and people to the team. It's this second part where the cone of uncertainty comes in helpful for them. You may have heard the adage "adding people to a late project makes it later". This comes from ramp-up time. Adding people to a team or teams to a project slows the project down for a while before it speeds it up, so in traditional project management you usually don't know a project is late until you're too close to the deadline to overcome that slow down. Effectively leveraging burn-up charts and a cone of uncertainty allows you to see that your project is late much sooner - maybe in time to effectively add people or teams.
It is also worth noting that I've never known a successful organization go into a project (and estimate) cold. They either invest a fair amount of Architecture and Design time (I don't know if it's 30%, but it's usually substantial), or they're selling an out-of-the-box solution where they're leveraging past work for this same purpose. In other words, they're investing time and money they hope to recoup later.
Cone of Uncertainty Without a Fixed Scope / Fixed Deadline
This is probably the most common approach I've seen with companies that do contract work. Instead of one big commitment, there are a series of small commitments, like working for 2 - 3 sprints, or, if you aren't using Scrum, maybe a month at a time. I've worked with a number of real companies that do this. The plan might be for an 8 month project, but there are regular checkins and the customer can pull out whenever they like because the teams are delivering usable working software, so it's never a matter of cutting losses. Here, the Cone of Uncertainty sees huge value as you can make forcasts about when certain work might be done and also use those forcasts to pivot on which work is more important.
Cone of Uncertainty with No Commitments
This approach is far more common where the team doing the work is internal to the organization, but I have worked with groups where they are basically on retainer to the client and can use this model. In this approach, the teams have a few measures of success like uptime, reduced manual work, etc that they use to prioritize work. They work with the client to move the needle on those measures. This completely abandons the project model and therefor you don't see burn-up charts and cones of uncertainty coming into conversations as much. However, they do pop up from time to time especially on long-term efforts. For example, I might think that a platform upgrade is going to boost all of those measures a lot, but it will take a few months. Three weeks in, I can use this cone of uncertainty to see if that three months is panning out the way I thought. Maybe it'll take a lot longer because the new version of the platform is just a mess, and choosing to table it while the vendor works the bugs out is a better business decision.
Finally, Your Circumstance
Of course, I don't know what your circumstances are, the type of work you do, or how you write your contracts. These are ways other companies have leveraged this model, but different markets in different countries are more or less open. Further, even if the market runs a certain way, different customers in that market will be all over the place on their tolerance for commitment. When companies try to shift to this approach, I see the most success when they find a customer who is open to try it rather than force it onto a customer and look for a team that volunteers to work this way rather than force the team into it.