Depends on how you define reasonable!
For Hitchin and Stevenage it's currently three trains per hour, at intervals of 15, 30, 15, with some of these missing at arbitrary times throughout the day. Then you have the issue that the timekeeping of the northbound service is erratic thanks to the Thameslink effect, which even with this level of frequency means you could well be in store for a quite lengthy wait on a core platform if something is amiss.
The alternative is to head to King's Cross, where at least there's more options for things to do if having to wait (e.g. cafes and restaurants), plus the chances are that the train will be waiting in the platform prior to departure so one can at least settle in to the train rather than having to stand on a platform or sit on a metal bench - not that an Undesiro is much better of course!
Add in the prospect of the 365 services at certain times and it's easy to see why people might prefer to opt for King's Cross. Any core station other than St Pancras is simply a gamble as if something goes wrong one hasn't got access to the stopping service.
Naturally the scales may tip slightly when (if!) the 700/0 services run through to Sevenoaks / Maidstone, and the second Cambridge/Brighton service would certainly even things up. That service will definitely take people off the up stopping service.
As to whether the above ThamesLink/ Lottery constitutes a reasonable service, I'll leave that to readers to decide for themselves...
Meanwhile, let's think back a couple of years. One would turn up at King's Cross, check the board, head to the platform, where a 365 would be waiting, comfortable seat and table whilst awaiting departure, then a fast and comfortable journey to destination. No fuss, no bother! (And certainly didn't run the risk of taking 50 minutes to get to Finsbury Park either...). The only thing which might have been nicer in those days was no 4-car working, which we could have had but for platform extensions at Meldreth Shepreth and Foxton and a few extra units in the fleet.
Totally agree with this. Waiting for an inevitably late running train downstairs at St Pancras isn't a pleasant experience, especially with the platform staff screaming over the tannoy for passengers to stand behind the yellow line.
Compare and contrast to Kings Cross where the whole set up described by @bramling is far more civilised.