Thats not how the economics would work. More liquid nic for people who buy things would increase the value of goods and therefore raw materials. Its inflationary and would therefore do nothing to the economy long term but reduce the value of liquid nic held now.
If it costs 20 million for a mech do you think that mineral prices would remain constant?
In addition what it would do is set a value on work. High end players would compete for plex's setting a value for them on the market that would be roughly equivelent to the level of work needed to gain that nic.
Its really important for a game to have this - right now there is nothing setting the standard and as corps accumulate goods faster than they are destroyed the market deflates and therefore devalues the raw materials making it harder for new people to compete.
The only thing that would become hilariously undervalued is missions rewards.
Its pure 1984 Orwellian economics at the moment. The main driver behind prices and market action is a surplus of production compared with use.The trick then is to provide a method of redistributing this surplus to the lower end of the market. This will be achieved by plex's as the high end players rather than working to buy plex's will to a certain extent just use the reserves they have that will be worth more as time goes on. Only once these reserves start to dwindle will the balance of work return to the higher end players. I'm talking of course as a populace, some corps will continue to overproduce.
The only thing a plex would change is the attrition of high end corps and players. It provides them with the ability to play for free and more than that provides industry with a reason to exist outside of pvp. Market pvp becomes a part of the game again.
Another point to note is people talk about the moving of nic around and that nothing is lost of gained. Contrary to this in fact it will be impacted by the attrition rates of new players and old players. Will the nic left on the accounts of new players who have left be greater or less than the nic on the old players who leave.