There are multiple reasons as to why there is no such thing as an unbannable script/client and I'm not going in depth about every single one of them, but there's a few main ones.
1) Changes and updates
The runescape clients is being updates every week, bots and sometimes scripts have to adjust on these changes, the problem here is that we don't always directly know what they changes. Keeping things the same as we did could already be a way to get detected since hey look at thing differently now. There's an information gap between us and Jagex.
2) The 'system' isn't the only thing that gets you banned
Even in the impossible scenario of the client and script not being able to be detected in any way, there's still going to be people seeing you, reporting you ingame or even tweeting a jmod. Goodluk writing algorithms for that
3) All codes can be broken
Sort off ties into point number 1, even in the impossible scenario of an unbannable script+client combo, you have no guarantee that it will still be unbannable after the next update. This is even more true because this is java, it is relatively easy to obtain a pretty clear view of the source code of any client or script, once you have an idea on how the source code works, you can get an idea on how to detect patterns/algorithms on that source code, not matter how random or human-like it is.
There's a few more reasons but I think Ive given enough of an explaination as to why there's ALWAYS going to be a risk in botting, you can lower that risk obviously, but it will always be there.