You also benefit from the tailwind of the CitusData team making continued improvement to the extension, whereas an in-house system depends on your company's ability to hire and retain people to maintain + improve the in-house system.
It's hard to account for the value of benefits that have yet to accrue, but this kind of analysis, even if you pretty heavily-discount that future value, tilts the ROI in favor of solutions like Citus, IMO. Especially if your time horizon is 5+ or 10+ years out.
Like you said, if they made this decision 3ish years ago, you would have had to be pretty trusting on that future value. A choice, made today, hinges less on that variable.