Pros and Cons of Platter Mass

I would be loathe to do anything to dramatically alter platter mass or the materials used at the bearing/thrust pad interface, or even the lubricant, without conside   ring the effects on the other constituents of the closed system.

I wish more folk would heed this sage advice.

I have seen a number of Garrard 301/401 TT's with aftermarket thrustpads/balls that have damaged/worn the spindle.
I have seen many TT's with accelerated bearing wear from using "better" oil.

I have learnt that the Thrust Pad can show signs of increased wear after a lesser period than anticipated, where inspection of the part has shown noticeable indentation occurring from the Point Loading.
There are harder Thrust Pad Materials that will alleviate this condition and offer improved performance and longevity of use.

And you forgot to add may well stuff your main spindle.

By using a "harder" thrust pad you increase the chances of wearing out your spindle - yippee - try finding another spindle for that vintage turntable that is no longer manufactured. The improved performance will only last a year or so until the spindle is stuffed.

The best direct drive motors do not require as much mass as the best belt drives.

No.
If you look at the Technics SP10 series - the most powerful SP10mk3 has a mucher higher platter mass (25lb ) relative to the weeker motored mk2.
Similarly with the Kenwood L07D - highish platter mass was oart of the design - the argument was high inertia, resistance to stylus drag.

Finally the most powerful direct drive motor built by Technics for lathes - the SP02 - dwarfs the motor in the SP10mk3 - but it was specified to be used in the Neumann lathe with an added 70lb flywheel.

If you build a platter out of weetbix, it will still sound like weetbix irrespective of drive topology.

It is becoming more common for manufacturers to use a magnetic thrust mechanism. No ball and no thrust pad.

And it is a stupid idea - you cannot accurately measure the groove when the platter is not grounded or referenced to a fixed point relative to the tonearm.

Same as air bearings, platter is not grounded, and it is not possible to accurately measure the groove.

@totem395 
here is an example of high mass platter speed stability -
https://www.youtube.com/watch?v=IkrBPunk2rI

@totem395 
You are looking at this upside down -
the objective of a high mass platter is to make it immune to stylus drag.
How heavy does it need to be - Brinkmann says 15kg minimum, Kuzma  claims similar.
Kuzma says stylus drag cannot be eliminated completely - so it boils down to heavy platter, high resistance to stylus drag vs light platter less resistance to stylus drag which would require fast and smooth non invasive speed correction.
So it comes down to which is the lesser of the two evils - very small stylus drag effects on a high intertia platter, or larger speed errors corrected more vigorously on a light platter.
Unfprtunately cost always factors into design high mass platters with sutiable quality bearings are not cheap.