I think the benefits of trustless shared masternodes would outweigh the downside...All DASH locked in the savings account would be pooled together, allowing for individual terms to end at the same time as taking down the least amount of masternodes due to the reduction of pooled collateral. People who want to run masternodes but don't have the 1000 DASH, could subscribe to running one with a minimum of 200 DASH or so with the remaining 800 or so required counted from the savings pool of all locked DASH.
It sounds neat.
But the main problem is, nodes simply aren't a significant economic hub in crypto networks in the first place. They're not money-making commercial enterprises but rather a very cheap to maintain component of the network. So the issue of whether they're shared or not is irrelevant IMO compared with the viability of the reward they receive in the first place.
The use they have in Dash's case isn't as a revenue-earning business (at least not primarily) but to boost the capital value of the mined currency by decoupling its mining from the services layer. A bit like how a steering wheel is an extremely cheap component of a motor car - maybe only $100 in a $20,000 vehicle - but without it the car's value would be nothing. You wouldn't be able to drive it.
All the same, pretending that the steering wheel should be valued at $20,000 as well would put the car's value at $40,000. So out of whack with any competitors that you wouldn't get any customers.
So it is with masternodes. They are an extremely cheap component that requires us to compromise very little in comparison to bitcoin to gain a huge advantage. If we pretend however that they are so expensive to run that it requires half the entire coin supply to support, then we'll lose all our customers. (As we have done). Shared or not-shared masternodes is an irrelevance.