Its a car boot sale, what did you expect the guy to do... of course he would haggle.

The question is: do you come out in flying colors.
If you are a good pro, changing system will be a financial bump on the road, and a bit of time with a new dslr for expediency when on the field.
You wont loose any skills by changing brands, so no worries there.

Now if Pentax does not offer good options for what you want to do, then look for what works for you.
Pentax stops no one from being a pro, some types of photography might be more difficult though. Then again others might be easier.

Something has already worked for you. If thats what you wan to continue doing, then see why it has worked, and don't waste too much time wondering about other greener pastures, they will always be there.

I do not know if this is the reason why pentax has opted to go for wider lenses first, but consider the uses of long lenses.
Sports (cars, footbal, etc), Wildlife photography.

For the case of fast primes in this range 300mm F2.8, 500mm F4, etc we are talking about prices which only professionals and a few amateurs are willing to pay.
With faster AF tried and tested by professionals for many years and better high ISO on the top of the lines of Canon and Nikon, a professional will probably choose those other options.

Now this might warrant a delay in producing theses lenses until a very good package of camera and lens performance is produced, but it does not prohibit producing zooms like the big sigmas or more modestly fast long primes.

The K-7 seems to be geared very much for outdoor use, maybe it is the first step, followed by a 400mm or a long telezoom. After lessons are learned from Pentax's first iteration of SDM, more expensive lenses can be produced.

If the person cannot tell you what mount it is, ask for a photo of it.
Any KAF will work fine in either the MX or K10D.
At 72mm, maybe its the 28-105mm F2.8-4, thats the more common lens with 72mm I can remember.

I had the DX F1.7, very nice lens, plenty sharp with good bokeh. Small too. No real advantages on my SMC-A, except for all metal construction.

"I shoot people for fun"
"I shoot people on the street"
Better put a little camera icon there...

Read the posts, all perfectly valid and reasonable.
If you must be evangelical about your brand, you can say you can get get FA* longer lenses which are pretty good, or weather sealed DA* up to 300mm.
There are also third party makers which I'm sure Canon and Nikon users also buy from.

IMO, don't bother trying to turn them over to Pentax, its not going to happen, not based on rhetoric at least.
Enjoy that forum and learn from it.

I have this one, and its the best 2x tc I've tried. All tc's will reduce some IQ, but this one is not too bad. I also got a quantaray 2x tc (which I suppose is the same as the sigma, usual with quantaray) and in terms of sharpness I see no discernible difference except the tokina gives more purple fringing (nothing to bad).

Idiots will never go extinct...
Sorry to hear about that.

Fast probably was a term that originated from those photographers who bought them for the ability of using faster shutter speeds they required.
I've also heard bright lenses, maybe coming from the desire of having the ability of more easily having a good view in the viewfinder or allowing easier photography in low light.