Arm doesn't only do ISA. It essentially wrote the standards for the AMBA/AXI/ACE/CHI interconnect space. Standardizing chip-to-chip interconnects is very much in Arm's interests. It is a double edged sword though since Chiplets will likely enable fine grained modularity allowing IP from other vendors to be stitched around Arm (eg RISC-V IOMMU instead of Arm SMMUv3 etc).