Absolutely correct. I do not dispute the point that Hokkien should be written as it is spoken, on its own terms, and not be dependent on Mandarin. God-forbid, using Mandarin as a crutch for representing Hokkien is the last thing on my mind - much less suggesting that Modern Standard Chinese is the New Testament of Classical Chinese! 吐血、吐血...
What I meant is this: I believe Sim is trying to pick up a decent number of Chinese characters as a means of reading our 唐儂字 Tng-Lang-Ji Hokkien conversations. Now, he could be waiting till Kingdom come before the Web finally has a full plethora of Hokkien texts written in a universally-accepted Chinese character standard before he has a sufficiently-decent resource - if, at all, that will even come to fruition. So, in the interest of expediency, what is he left with at present? Yup... a Web-full of texts written in that ghastly language called Modern Standard Chinese. It's not ideal, I know, but still better than nothing. Of course, between that and Classical Chinese, I would anytime rather Sim use Classical Chinese as his base.
Put it this way: Hypothetically-speaking, if Sim was learning the character 不 today for the first time, I would not want him to see it as “the Mandarin word bu, read in Hokkien”, just because 不 occurs in Mandarin speech, whereas Hokkien uses 毋 m to denote the negative. No, no, no. I would much rather he learnt to read 不 in Hokkien terms as plain and simple 不 put4, a word that occurs in more literary aspects of Hokkien and Literary Chinese. And where would it occur in common Hokkien speech? Well, there's 不孝 put4 hau3 and 差不多 cha1 put4 to1. Similarly, if he saw 吃, I would not ask that he 訓讀's it as ciak8. No, no, no. 食 is ciak8, 吃 is khit4.
I also note and agree with you point that while Hoklo has a certain relationship with Classical Chinese, they are not the same. There are sufficient examples of words and morphemes of non-Sinitic origin in Hoklo to make my quest for an all-Tng-Lang-Ji-written Hoklo a totally futile mission.

Of course, all this is on the assumption that Sim does, in fact, want to learn to read Hokkien using 唐儂字Tng-Lang-Ji, and in no way am I forcibly prescribing it to him as the way to read Hokkien.
** Sim - My sincere apologies for drawing you out as an example, and I hope you do not take it that I am being condescending in any way.
* * * * * * * * * * *
Now, regarding Cantonese...

Okay, I should qualify my previous statement. I do acknowledge that the current Hong Kong Chinese education model of teaching Chinese using Modern Standard Chinese as the lexical and grammatical base for reading and writing does have an impact on the masses' vocabulary and word usage. But I see the impact as one that is more of supplementation rather than displacement.
For example: The Hong Kongers knowing how to read and write ‘just now’ as 剛纔 kÔng-chŎi does not mean that they have stopped using the Cantonese colloquial 頭先 thău-sîn in everyday speech. And I would bet that implicitly, they would know that if they had to write the specific words thău-sîn, it would be 頭先.
What I will acknowledge is that it has not been perfectly zero displacement. With 給 in the standard written lexicon, almost all Hong Kongers have no idea that their colloquial péi ‘to give’ is written as 畀. They either write it as 給, or that horribly-incorrect 俾. Now that is what I do not want to see happen to Hokkien. hO7 is 予, not 給.
And in some instances, Hokkien speakers have it better than Cantonese. At least for ‘things’, we know how to write miq8 kiaⁿ7 as 物件; the poor chaps have to resort to the borrowed 嘢 for yae.