I think Open source LLM's will hit a ceiling for this one reason: safety guardrails.
Today we see Mythos and GPT 5.6 Sol put under heavy scrutiny for the primary reason that it is too unsafe to release to the general public.
The guardrails come in three layers - safety baked into the model itself, immediate flagging and offline batch analysis.
Here's a strange example from Sonnet 4.5
Mythos was so cautious that you were not able to ask it a single question mentioning mitochondria.
I don't have an example ready but imagine FBI knocking on your door because you have repeatedly asked ChatGPT suspicious questions which were kinda valid individually - this is not hypothetical, something like this has happened in the past.
The "unsafety" comes in a few dimensions
cybersec capabilities that allow models to attack software in the wild and create space for hackers to do their thing
psychological problems that have long term cultural consequences like: enabling AI psychosis, CSAM, Gore
bioweapons and nuclear - AI can help bad actors synthesise weapons that enable disproportionate damage
Perhaps there's another layer of "unsafety" which is really just China and other state actors getting access to intel for either economic or war reasons.
Open weight models fundamentally don't have access to these guardrails. The only safety guardrail it has is the first one - baked into the models. Therefore, my claim is that they will either hit a ceiling or forever be meaningfully behind closed models. This limitation will severely hinder the performance/accuracy.
Either I'm right about this or all of the safety rhetoric was always theatre or an excuse to limit China's progress.
A third option is that we will still see progress in alignment such that level 3 (offline) and level 2 (immediate) guardrails can be collapsed into level 1 (model weights). It is not yet clear whether this is possible - my guess is that it can help but not as much.
Open weight models will never be state of the art. You will always have "trusted" model providers that will do all the checks and safety guardrails and you will always have open weight models lagging behind meaningfully.