I'll start by saying that HWE is absolutely the best way to flush the greatest quantity of soil from a carpet in a single shot!
That being said, if a commercial carpet is well maintained on a regular basis with a good encap system - HWE can be pushed back pretty far. We have many accounts that have not seen HWE in years. Yes, yes, yes, I know the rhetoric... the mills recommend that HWE should be done at regular intervals. It's generally suggested that the carpet receives HWE on a yearly basis. That's certainly an excellent suggestion. However if the carpet is WELL MAINTAINED with a good encap system, there may not be a need for such frequent HWE intervals. So close monitoring of the carpet should be part of the equation.
With a good encap program - HWE can effectively be referred to as the "INTERIM METHOD". The carpet can be well-maintained on a routine basis if a good encapsulation program is in place. And then on an interim basis, HWE can be provided as needed. As many professional cleaners are seeing, if the encap program is a good one, the intervals of HWE can be extended.
If you plan to rely on encap as your main system for commercial maintenance you'll need to routinely evaluate the carpet to determine that the carpet is in good condition and that the soil is being managed as intended. You should plan to inspect the carpet closely all the way down to the backing. You will also still need to have HWE at your disposal for flushing any bio-contaminants that may occur. And you'll need to use HWE whenever the carpet needs an interim flushing. A good encapsulation program along with occasional HWE whenever it may be required is a good way to go for commercial carpet care.