There is no hard limit to the output power from a flyback topology. It's a matter of which is best for a given situation. One could create a 1kW flyback, but it would not likely be economical. This is a business where they have blood-on-the-carpet meetings over 3-cent diodes and recognize that it is cheaper to hire another full-time engineer than to put an extra few pennies of cost into their product- so not picking the best topology for the requirements could foreshorten one's career.
The flyback converter uses the core less efficiently (means more money, size and weight for a core, which matters more as power levels go up). As Russell points out, the flyback stores the transferred energy in the inductor, and releases it to the output, as opposed to most other types that transfer energy when the switch is on. That means necessarily the current stress must be higher, since all the energy is being transferred by a single switch, and it can only be on a part of the time. (Keep in mind that some losses are proportional to the square of the current, so 10A for 33% of the time vs. 3A for 100% of the time represent the same load power, but the resistive losses in the low duty cycle switch are 3.7 times higher.
The voltage stress on the switch in a flyback is far higher (double input voltage) compared to a two-switch forward converter (just the input voltage). This makes the switch more expensive, especially for MOSFETs, where chip size (and therefore cost) rapidly rises with voltage rating, all other things being equal. Switches that are less sensitive to voltage (in cost) tend to be rather slow (BJTs and IGBTs), so again less suitable for flyback converters because they would require a bigger core.
Flyback converters have a number of advantages (potential simplicity because of the single switch, no output inductors required because the leakage inductance works for you, wide input voltage range), but those advantages mostly dominate at lower power levels.
That's why you'll almost always see flyback converters used in AC adapters, and you'll never see it in a 250W+ PC power supply-- both applications where any excess cost that is safe to squeeze out has been squeezed out (sometimes more that that!).
Your design is good. It is OK to use 50 Hz isolating transformer and then buck DC-DC stage. However: you should take care of interference and isolation. This may be not so easy, if you do not have good enough PCB technology at hand.
You can not make your buck stage on one IC because of high voltage / power requirements. You can use ready-made buck controller (for example, LTC3810) and two small power MOSFETs rated for 80 V operation. LTC3810 can regulate its output in range from 0.8 V to 50 V. You have to select the proper feedback correction to run the switching regulator in such a huge output voltage range.
Additional benefits of this design are: built-in over-current protection and soft start.
The problem is: you need a good PCB technology to make this design work fine.
As for the switching noise - it is only a matter of proper filtering. I have designed a lot of very sensitive analog and RF circuits, all containing switching regulators. It is enough to filter the output by one or two stage LC filter with MLC capacitors and low quality factor inductors. I suggest to add one common mode choke on output of your DC-DC stage and another one - on its input.
The linear regulator after the switching one does not help against a switching noise, it is a myth. You can check specs for linear regulators for AC suppression: it is pretty low above 0.5 MHz. And we are talking about noise / interference from the switching frequency up to 100 MHz, at least. So cheap passive components (inductors, ferrite beads and MLCC) solve this problem.
The only drawback of using 50 Hz transformer compared to fly-back switching design is: the capacitance between Mains and your DC output is ~100 times larger with 50 Hz transformer. It may cause problems in RF applications.
Once again: your major problem in this design is the PCB technology (2 layers is absolute minimum, 4 is good) and proper PCB layout design.
Best Answer
There is no inherent limit to the power you can get from a flyback topology. It just happens to be more cost-effective for relatively low power applications. Many flyback converters control the peak current by a low-value external sense resistor in the switch source (or emitter) circuit.
I suggest auditing the Coursera courses on switchmode design (from the University of Colorado, I think). They're free to view (but unfortunately not the quizzes).