wa 'alaykum salaam
This is a silly issue to continue as both of you are not really contradicting each other.
What is being questioned is the fact of its rudeness. That is the point trying to be gotten across. Al-Kashmiri never said not to thank, rather he said it was a good thing, and you akhi also did not say not to thank, therefore you are both in agreement. However what is being questioned is:
Is it really rude to not post thanks?
And in this I agree with Al-Kashmiri. We're not talking about whether it's good or not, rather the opposite of if it's rude to not do so. And in truth it is impractical!
Think if I'm here as a guest, which I am liable to do many times, and I see a good article; I love it dearly but should I be obliged to log in just to say so ?! Just to come in and say "Masha'Allah that's a good article - Jazakallah khayr!" ... or would it be better and more beneficial for the person if I say took this article and posted it on my newsletter so that many others can get access to it? Or what if I took it and gave the message to the rest of the family - thus increasing the khayr from the knowledge? What is being expressed in writing I feel is only a shallow appreciation and should not stand for a sign of politeness or rudeness.
It's a shallow judge of appreciation, and those who do not say it should not be blamed or regarded any lower than those who did. Allah knows best, maybe the one who read it but did not reply felt its effects more, learned more, spread it more, whilst the other merely scanned through it, and to make the poster feel better replied with a hearty thanks. I've noticed this as well.... sometimes when one posts an article which is entirely too long to be read in the span of seconds, you have people posting to say
! ... I would personally rather they read it, instead of having to thank.
These are just separate examples; others could include perhaps a User not having time to post a dua, perhaps they said it but did not write it etc - overall there can be many excuses why one may not have given thanks and for that its not blameworthy.
Also though we may claim we are here to post only for Allah, that cant be said of everyone. Everyone has a struggle with intentions sometimes, and so we should never be too certain of ourselves. I mean, even I will claim it does feel nice when one thanks you for their post,and it maybe saddening when you see that nobody does to another thread. But in reality as has been stated, its no scale and we should constantly strive to purify intentions, as even the scholars of the past did:
Sufyaan ibn Sa'eed ath-Thawree said, "I did not treat anything more difficult than my sincerity" -
(The Etiquette of Seeking Knowledge - Shaykh Bakr Aboo Zayd)
Therefore, to encourage that, I dont even see the need to ask others to ask. In short, even the one recieving the knowledge does not do it because of the poster, but via the poster - however it is from Allah that all knowledge comes. Therefore we should thank Him first and show Him our gratitude by improving ourselves by whatever we come across. If that includes showing gratitude to the one who posted, that is good, but if it can be achieved otherwise I feel it does not have to be a critisized factor.
Also though we post in public, in reality the best supplications are those for your brother/sister in secret, so that too should be kept in mind.
Showing gratitude is good, but mandating it I feel is not.
May Allah correct all our affairs
wasalam