Can a Buck converter operated at 200kHz be used to step down 230V DC input to 5V DC output?
Yes, it is possible, but not the best option.
In a buck converter, you have either two FETs (synchronous) or one FET and one diode (not synchronous). Both components must withstand the full input voltage plus some margin, and also the full output current. So if you want 5V 1A at the output, you'll need FETs and/or diodes that can switch quickly enough and withstand say, 300V and 1A with good efficiency.
On the other hand, in a converter based on a transformer, the primary side transistor(s) only have to handle the primary current. For a 5V 1A output, you'd have about 20-25 mA primary current at 230V DC.
Switching losses are proportional to the product of current, voltage, and switching time. Therefore when there is a large input/output voltage ratio, using a transformer minimizes switching losses by running much lower current through the switching device. It also allows to use lower current devices which are cheaper and require less drive current, further reducing losses. Gate drive losses are important especially if the driver is powered from high voltage through a linear regulator, which dramatically multiplies losses.
Then there's ON-time. With a ratio of 5/230, duty cycle will be 2.1%, at 200kHz that's an ON-time of about 100ns. Low enough to be impractical. It will spend almost as much time switching as it will spend being on, which means switching losses will really be a problem.
And you need a high side driver for that FET, which means more complication.
A transformer will operate with a much more comfortable duty cycle, and the switching device is low-side, easy to drive.
The inductor also plays an important part. In a buck converter, it carries the full output current. Also if you want a reasonable inductor current ripple, say... for 5V 1A let's go with 500mA ripple... and you decide on a more realistic ON-time like 300ns, lowering the frequency, then you'll need a somewhat large inductor that will take the full current.
That's why transformers usually work better for high input/output ratio power conversion, even if you don't need the isolation.
Note the same applies to boost converters.