Those two terms are related but definitely are never interchangeable. Market share is the portion of new sales a company is getting. Install base is the portion of existing in-use products that were from that company. Install base is essentially market share integrated over time, less systems that are discarded or otherwise taken out of service. If market share never changes, install base will approach the same proportions but it's a lagging indicator.

Sure, but if the point is showing how Intel isn't really in such a bad spot as one might think just looking at the install base would be pretty deceiving and semi-meaningless.

Of a market that is dying between two growing at the edges. Mobile and server clear trump personal compute. This the markets devaluing intel.

To be fair it's not like there is that much profits in mobile either. ARM CPUs are almost a commodity and the margins aren't that great.