Several of my bigger clients have in-house security system techs. All techs were previous employees of integrators.
Primary driver for client's is the speed of service response, and the tech's familarity with the facilities being serviced, not cost savings. Most clients continue to use integrators for new installations and major upgrades.
One integrator I work with has a particularly sweet deal where the client pays for an annual service contract, yet the client's in-house tech handles about 75% of the calls. (Client still likes the warm and fuzzy feeling of having a service contract in case there is a major problem.)
I have found that techs who work with just a couple of products day in and day out become very, very good - far better than the typical integrator tech who typically works with dozens of products in any given week. I knew of a Casi-Rusco (RIP) engineer who used to call one of my client's techs when he needed advice on some of the nuances of Micro5 panels.