Modular Monoliths for High-Throughput Financial Systems: A Viable Long-Term Strategy?

I've really been thinking about the modular monolith idea, especially since I saw it used in some super complicated financial systems. Think like a derivatives exchange. I love the concept of having clear boundaries for each part of the system but still deploying it all as one thing. It just makes things easier to manage and keeps complexity in check, especially when you're just starting out. But what happens down the road? What about scaling and performance when you're dealing with tons of transactions? Do the perks of easier deployment and faster development make up for the bottlenecks you might hit as the system gets bigger? When does that 'monolith' part, even if it's modular, start to mess with scaling up individual pieces that need totally different resources? Or, maybe we're just not giving enough credit to how well a good modular monolith can actually scale horizontally if you have the right setup.