A few things:
• The drop in sales could be fallout from McFarlane no longer being on the title. That takes a few issues to have an effect, and his last issue was 328.
• Claremont, as a writer, would have had less control than an editor
• While I don't disagree that a lot of direction comes from outside the Creative department, I don't think that kicked into high gear until a few years into the '90s, when every character that had an ongoing suddenly had a family of titles (Thunderstrike, War Machine, an explosion of X-books, etc.)
• Since McFarlane's art was certainly a very visible reason for the popularity of Spider-man, management may not have been as acutely aware of a demand for more Venom. I doubt management is reading letters from readers.
Read the link I posted. I'd say that Micheline's version of events is a much better indication of what happened than our speculation as to what we think happened inside the Marvel offices