TheR is right, the GP/Corsa was the base for the larger 500s, but it's still a platform dating back to 2002.
Before we get into the nitty gritty, I must state that I am by no means a final authority on these matters, but I like to follow market analysis and corporate news from the automotive world to keep up with future possibilities in the car market and regarding personal ownership. As such, a lot of what I discuss below is either observational or speculative as far as future trends are concerned.
Not having newer platforms in development would mean they'd fall behind the competition eventually. Newer more rigid chassis designs would provide better handling (not many have come close to replicating the Fiesta in this regard), more interior space (through better design of structural elements allowing for more interior volume), and a key aspect that's always shifting is safety (new small offset impact tests have been the bane of older platforms as they simply can't compete with newer cars that have been designed with these tests and crash circumstances in mind).
All things aside, as AB100 said certain aspects of vehicle ownership negate the whole discussion, otherwise we'd all follow the old Soviet one-car approach and be done with it. What worries me about FCA is that it seems they've poured money into their more elite brands (Ferrari, Maserati, Alfa), while the bread and butter Fiat has been regurgitating the 500. The strategy might save money for a while, but since Ferrari is a lower volume seller in the FCA holdings, it wouldn't impact the long term profitability of the company as a whole. Maserati is struggling to compete with rivals, mainly due to the fact that at that price point, people wanting a good looking "sporty" car aren't willing to put up with the compromises it comes with compared to, say, German or British rivals. The Alfa range has been a plethora of rebranded optioned and tarted up Fiats for a while now, and initial reviews of the new RWD Giulia (while raving about the return of RWD to Alfa) seem to indicate it's in no position to unsettle the BMW 3 series from the throne in that class. It didn't need to be a blockbuster and knock out the 3 series right out the gate, but it seems to fall behind on some of the fundamental expectations in that class of cars.
This all brings us back to Fiat. If Fiats sell well, FCA makes more money to spread around the corporate offices and other brands as well as returning the investment into making better Fiats and providing better dealership and ownership experiences (thus ensuring return customers). As it currently stands though, chugging along on the novelty of the 500 isn't a sustainable business model for much longer. For example, VW and Renault both have global modular platforms (MQB and CMF), and architecture from which they can derive everything from dinky cheap runabouts for emerging markets to large off-roaders. As such, they are in a position where a few months of development is all they need to fill a market gap and get sales. Fiat, lacking such an architecture, has to compete in a global market without an easily adaptable platform and therefore is likely to miss out on sales.
Where does this leave us? As dealers get more frustrated, odds are they'll turn to different brands. This in turn leaves us without dealers to resolve issues with, and buyers without dealers to purchase, all resulting in a vicious cycle in which Fiat sells even less cars and has even less money. As the end users, we might run out of support for our vehicles, or out of hope of finding new ones (Fiats) that we want.
