Lid or no lid, it’s still a problem if your drink falls off the tray table. Which is one of the reasons they say to put your tray tables in upright an locked position.
But even beyond that feeling like you’re falling out of your seat is unpleasant.
Automakers solved this problem many years ago, by making cupholders that are deeply recessed instead of thinking that cups wills somehow magically stay in place with a very minimal indentation (see the "cup holders" on the backsides of glove compartments in 1970s American cars).
Commercial aviation could stick as many as 850 passengers on an A380-800 in it’s maximum configuration, but the most an airline actually gets is 538 because of business and first class passengers having more room. Increasing passenger space by even 1 inch or just a pound of weight per seat makes a real difference.
