You apparently have no (or very little) hands on experience with null meters, the 845 is indeed quiet, noise is under 200nV on the 1uV scale, 250nV 1mV to 300mV and 300nV above that, granted that is with a shorted input but that is a standard method of noise measurement. The Keithley 155 has very similar specs to the Fluke. While it is possible that FETs may produce lower noise, that is not always the case, bipolars can produce very low noise levels as well. What digital instrument that could possibly have a 1uV full scale range compete with a null meter for noise or accuracy with <200nV noise?
Best practice is to use the lowest possible noise levels in instrumentation to begin with, it is poor practice to use higher noise levels and then attempt to mask them by filtering and math which has their own problems, you should know this hard and fast rule and if you don't, shame on you, if you do and don't practice it, shame on you, digital is not the end all fix or solution, never has been and never will be. I have used null meters for decades, I'm quite familiar with their ins and outs, until you have some actual experience, don't knock them, they do their jobs exceedingly well.
The input diodes are for clamping overloads, they are inside the feedback network so no the input impedance does not fall to 150K, the lowest input impedance under 'normal' input conditions is either 10 Meg (older units) or 1 Meg (newer units), similar conditions exist in the 155 units. Who cares, at null (after all this is primarily a null meter) there no current (or very little ) flowing in the input so your argument is again incorrect. So an input overload causes a few seconds overload recovery, what happens in a ADC when its input is overloaded and needs to change scales? That doesn't happen instantaneously now does it? Correct use of a null meter is to start out on a higher voltage scale (when the input voltage isn't known closely) and adjust the scale downward so that a null can be achieved. If you are silly enough to put the meter on a very low scale with significantly higher input voltage, guess what, the limiting diodes cuts in and shunts the current away from the input circuits, just what it is supposed to do. Virtually all of your arguments start out with the misuse of the meter, what do you think happens in any other instrument where its input circuits are overloaded like you are describing? The same recovery scenario takes place.
With all respect due, as to your "suitable scaled and suitable filtering and statistical info", baloney, you're already adding more noise, error and uncertainty to the mix, again you start out with the lowest possible noise, no amount of filtering and statistical dithering is going to improve on that, you claim a null meter is slow, I don't think all of your add ons are going to be all that fast nor as accurate, every addition to the signal chain introduces more noise, error and uncertainty, that is a fact of electronics . Your assumptions are obviously not from actual use or practice, your arguments are mostly specious, and averaging by eye, I do it when needed, works just fine and is quite accurate, your digital is still incremental and has inherent limits, it is not best practice, just ask the labs who still use both old and new technology, there are some huge companies out there that rely on both technologies and know when to use the correct instruments. I know primary cal labs that still use null meters, are they all possibly wrong in their conclusions?
Yes I have used DVMs as null detectors sometimes, their limits are quite obvious compared to an null meter in actual practice, within certain limits, a DVM can be used without problems but when you're in the hunt for PPMs or lower, a good null meter is exceedingly hard to beat. Just check out the manuals for instruments that measure PPM or lower, their calibration setups require null meters, not DVMs.
Get a good null meter and learn how to use it properly then come back and talk about them, experience and use count for a lot when talking about an instrument, just looking at schematic and making observations is not best practice. Try talking to others who have used null meters for years and get their opinions, it isn't just myself or MisterDiodes who share these opinions.