We can see from a relatively simple analysis that today's absence of inert-gas turbine generators has nothing to do with technical feasibility.  It is soley a matter of economics.

How does a nuclear heat source change the economics?  Comparing to the points above:

1. One cannot buy a steam turbine operating at 650°C and higher temperatures.  The most feasible option for taking advantage of the high temperature of molten-salt and pebble-bed reactors is gas turbines.
2. The hot-side heat exchanger is either inherent (in a gas-cooled reactor) or required to separate the nuclear materials and the working fluid (molten-salt reactor).
3. Nuclear plants do not chemically modify the working fluid of their heat engines, so are the equivalent of "external combustion".
4. The cold-side heat exchanger is required (like the condenser in a steam turbine).
5. The reduced operating temperature is a given, set by the nuclear heat source.
6. Since the gas turbine can operate at a higher source temperature than a steam turbine and can thus achieve greater thermal efficiency, it improves the return on the capital investment in the rest of the plant.  This reduces costs relative to revenue.

The thermodynamic properties of inert gases are well-understood.  Designing a fractional gigawatt gas turbine to run on e.g. helium would require design changes such as gas bearings (to eliminate petroleum lubricants or water which would cause corrosion or coking in the hot side), but these have already been proven in other applications.  The only reason we aren't running helium turbines today is that it would increase both capital and operating expenses.  If the heat source was a high-temperature nuclear reactor, the helium turbine would generate more revenue than a steam turbine for the same capital expense in the reactor.  This is why we can expect to see inert-gas turbines as part and parcel of any Pebble Bed Modular Reactor (PBMR) or Molten Salt Reactor (MSR) powerplant.