.... Do you think henceforth the MP will update according to when Xeon processors get updated which I think is about every 1.5 years?
First, Xeons in general update on wide variety of schedules. Xeon E3's update yearly. Xeon E5 class (which the Mac Pro will probably use) is pretty close to 1.5 on "tock" ( microarchtecture updates ) and closer to 1.25 on "tick" ( process shrink focused ). Xeon E7 is on an even longer cycle.
If Apple gets disappointed in Mac Pro unit sales the none of the component parts is going to drive the update cycle.
But from a Xeon E5 supply perspective I doubt Apple is going to start on the "bleeding edge" release schedule. For one, Intel seems to have gone to slow-roll releases of E5. A subset of the product line up is rolled out in some limited volume and full ramp takes about a Quarter. Second, at least for this next update, Apple has to turn around a new socket/chipset change in a year ( as oppose to the other vendors which largley used E5 v1 (Sandy Bridge) designs in 2013 and could spend practically the whole year on prepping for E5 v3. ). If there are new GPU cards also that's 3 custom boards on a short design cycle. For a vendor that is shipping out subtsantive firmware fixes on day 1 for their new product, it seems doubtful Apple has fully ramped up the "version 2" design staff and resources at this point. Same short staffing that likely slid the Mac Pro into late 2013 largely prep further slides in the future (at least for next of a couple of iterations).
I would be surprised if we didn't see a refresh in ~Q4/2014 as soon as Haswell-EP is available (because it is a very interesting CPU with features like AVX2, etc).
I suspect that Intel will only effectively "soft release" Xeon E5 v3 ( Haswell-EP ... why the code name when the product name is trivial to figure out??? ) in Q4 '14. It would not be surprising at all to see a slide into Q1 '15.
Of course, there's an argument to be made that given the focus of this machine, it should really be updated in sync with AMD's GPU release cycle, not Intel's CPU release cycle.
Rather weak position. Far more grounded would be that they are on roughly equal footing. If there is a major generational updates in GPUs and none in CPUs there is little reason to hold back on a yearly update. Likewise if the GPU is somewhat stalled and there is a new CPU then move.
AMD cycle is partially dependent upon their fab partners who can flake out ( looking at TMSC as a rather prominent example ). if the GPUs and CPUs drifted 9-13 months out of phase then Apple could use just the one whose phase landed in that product release window to get a next generation Mac Pro out on a yearly basis. The problem is the phase gap is going to drift over time.
The footing isn't exactly equal since the I/O chipset and CPU are basically bundled together. Every tick/tock cycle Intel is going to drive substantive CPU + I/O board updates which will trickle across much of the Mac Pro. The GPUs are far more self contained by the generational changes.