I don't see how Ms. Lien's shyness would be relevant to the way writers treated her character.
Perhaps because they weren't getting the performances they imagined? I dunno, but I do know several people who worked with her found her rather tricky to communicate with. Some actors and characters just fail to click with the writing team.
If there's stuff from long before her dismissal about the difficulties writing for her, then I'd believe it, but otherwise it just looks like rationalization.
As I mentioned, you can see them tinkering with Kes all through the series: her "elogium", the break up with Neelix, moving from gardening to nursing, discovering powers, pairing her with the EMH, then Tuvok, changing her hair and costuming, etc.
They really didn't have to rationalize anything about her dismissal. The producers had to let a character go in order to be able to afford to add Jeri Ryan. They narrowed down to the two characters/actors that caused them problems (be they discipline problems or writing problems, or whatever) and chose Lien to go when Wang got the "People" accolade. The writers say the character was hard to write for, and the many changes to the character over the years seems to bear that out.
Similarly, they let Gates McFadden go in TNG. Publicly, the character "wasn't working" and they recast for a different approach to the CMO. As we found out later, there was friction on the set caused by delays due to McFadden's very fine hair losing its style under the hot lights (hence the choice of expensive wigs when she returned), plus the eventual declaration from the actress that there had been friction between her and someone on high in production.
Similarly, TOS let Grace Lee Whitney go. Publicly, the character "wasn't necessary" and they wanted Kirk not to be tied down to one romantic interest. As we found out later, from the actress, there had been an addiction to diet pills, and ah... friction between her and someone on high in production.
OK, so the fact that they were going to keep Kes and drop Kim would seem to settle the argument about how Kes had to go because she was so difficult to write for. Had that been the case, she would have been the first choice to go.
No, it doesn't "settle" anything. Wang was also causing a series of difficulties - supposedly not turning up on time, despite warnings, not being on hand to do the off-screen lines when a dialog partner was on camera, and refusing to do his "homework" shadowing the director when he was pressing to be allowed to direct.
When Wang wasn't able to be let go, they then had to choose another. The writers could probably see plenty of story possibilities for everyone else, and less for Kes. They'd already tried a range of stories with her, leaving perhaps only Miracle Life-Expectancy Extension to go, in order to avoid latex age makeups. Lien was supposedly already allergic to her ear tips, which is why they let Kes let her hair down.