As it's a pair of competing commercial services, with brand new vehicles every 2 or 3 years, their very existence implies a need.
Oxford is in a fairly unique situation;
It has good road connections to London, and relatively poor rail connection - Look at Brighton, Cambridge, where the reverse is true: central London termini, frequent, fast, direct trains, choice of stopping patterns.
Crossrail is not currently intended to go near Oxford, preferring to terminate at low population centres either side of the capital, to ensure it makes huge losses. One day they may send it to where the people are, and there's no (real) reason why it can't include fast or semi-fast trains, so long as they stop at all stations between Padd and LStreet - At the very least, the Heathrow Express should be routed direct to the city (duh!). Similarly, the very situation you describe would guarantee a huge ridership from Oxford. But they don't call the ministry DaFT for nothing!
The Chiltern Line 'evergreen' extensions have historically been good value for money to build, and (almost unique) have mostly been built within budget, with most delays due to extraordinary circumstances, rather than the usual c***ups. The services normally offer fares some way below their "intercity" rivals (WC & GW).
The fact that you (and others!) choose to drive, and many thousands use the excellent, 24hr, economical, largely punctual and comfortable coaches only demonstrates that no senior civil servants in DaFT are Oxford residents.
Join us in Living In Hope that once built, Crossrail is taken over by someone with a brain cell or two

But don't hold your breath! :roll:
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Is Cambridge to London the same and do other towns or cities around London enjoy such services?
The only other coach commuting service of any size I'm aware of is Kent & Medway, where there's a variety of coaches, led by 'King's Ferry', a Natex subsidiary now). There, too, the rail service is abysmal, with semi-fasts almost a thing of the past, and A-B speeds slower than the final steam trains of the 1950s - but they have pretty abysmal roads, too, so the coach service is not so successful - and not so comfortable!
Natex did a trial commuter coach Milton Keynes - Islington - Docklands a couple of years ago; even one scheduled vehicle was not viable, and was discontinued after a few months.