RE: Accelerometer?
I don’t know if they do or not, but yes, you could easily calculate the mass at any point in the print.
While the weight of the bed is a static a easily pre-calculatable value, you cannot say the same for mass. And the latter is what matters the most, while your G-CODE can state that x will move 420mm with 10000acceleration it doesn't guarantee that printer won't override those parameters. That could work in theory if we'd something like an Apple ecosystem, where we'd had tight software/hardware integration and all parts calibrated and paired for one exact printer with all the downsides. Once any of those will became variable, process of precalculating it in software becomes practically impossible, so we need a hardware accelerometer to compensate for that.
If you think it's still anyhow possible, consider the table your placing the printer will have it's own weight and its resonance frequency which will change depending on the mass and what on it. 1Kg of liquid will behave differently that a 1Gg of filament lying on the table next to the printer. Even stepper motors will very their speed and acceleration depending on their temperature.
RE: Accelerometer?
One thing I haven't seen here from the people criticising the lack of accelerometers in the Mk4 is examples of poor print quality. Can anyone show the result of a print using input shaping on a modified printer compared to a standard printer?
Essentially, all the criticisms make logical sense and sound good in theory, but do those arguments actually result in a difference in practice? That's the real question and should be the first question. If the print results aren't up to standard, that's when speculation and theory should be used to find a cause.