I'm sure they don't want to end up like the Arduino ecosystem and be in a position where they have to spend all the money on the software development and other vendors undercut their hardware.
So they need to have hardware their competitors can't replicate, which means Broadcom or custom silicon. My guess is the latter is where it will eventually go, since they already dipped their toe in the water with the Pico.
Also the IO controller on the RPi5 is custom silicon
"RP1 is our I/O controller for Raspberry Pi 5, designed by the same team at Raspberry Pi that delivered the RP2040 microcontroller, and implemented, like RP2040, on TSMC’s mature 40LP process. It provides two USB 3.0 and two USB 2.0 interfaces; a Gigabit Ethernet controller; two four-lane MIPI transceivers for camera and display; analogue video output; 3.3V general-purpose I/O (GPIO); and the usual collection of GPIO-multiplexed low-speed interfaces (UART, SPI, I2C, I2S, and PWM). A four-lane PCI Express 2.0 interface provides a 16Gb/s link back to BCM2712.
Under development since 2016, RP1 is by a good margin the longest-running, most complex, and (at $15 million) most expensive program we’ve ever undertaken here at Raspberry Pi. It has undergone substantial evolution over the years, as our projected requirements have changed: the C0 step used on Raspberry Pi 5 is the third major revision of the silicon. And while its interfaces differ in fine detail from those of BCM2711, they have been designed to be very similar from a functional perspective, ensuring a high degree of compatibility with earlier Raspberry Pi devices."
So they need to have hardware their competitors can't replicate, which means Broadcom or custom silicon. My guess is the latter is where it will eventually go, since they already dipped their toe in the water with the Pico.