This is almost a solved problem; institutions are capable of sustaining intentions for insane periods of time. The major organised religious institutes have been pushing some basic standards for ethical living for millennia. There aren't a lot of projects that humans have considered worth a millenium of effort. Due to the limited abilities of humans that long ago they are all philosophical projects. But we can do it.

And on a smaller scale, institutions can keep grinding away far longer than makes any sense. A grant and a trust fund can last centuries. Look at the educational institutions like Oxford for interesting examples of maintaining a culture. It is pretty easy to imagine a group like that sustaining long term technical projects.

I don't think a millenium of sustained technological effort is actually that hard to achieve. We've obviously never done it before because industrial society is less than 1,000 years old. But assuming we don't self-immolate as seems likely then setting up ultra-long-term-projects isn't that hard to see.