and yet...
... IAS is not a speed.
Sounds ilogical, but it is not.
Note that we "would like" to have EAS displayed on the instrument panel, even though GS or TAS can be available, EAS is the primary parameter for flight. Due to some errors (position, instrument, compressibility...) we can only have IAS. But let's consider IAS as a close enough EAS, ok?
TAS is the velocity of the airplane relative to the air mass
GS is the velocity of the airplane relative to the ground
IAS is the velocity of the airplane relative to.... oops, relative to what???
EAS or IAS are not velocities.
EAS is the TAS that an airplane would have in the ISA at sea level if it had the same dynamic pressure that the airplane has at that moment.
Basically, the airspeed indicator is a dynamic pressure indicator, only expressed in knots instead of PSI or inches of mercury. Dynamic pressure is what we need to fly, after all.
If you lack TAS or GS you will have problems from the navigation point of view, but if you lack IAS, then you have problems to fly the airplane.