I agree, open source hardware (and especially open source silicon, where there are such huge barriers of entry) is very much different to open source software. It's possible that a breakthrough in direct-write lithography or similar would help to reduce these barriers, but it's not something we're betting the project on. This is one reason why our hope isn't to produce just one iteration of the lowRISC SoC, but to have a regular tapeout schedule. This means if you make a contribution, you know you'll be able to see it on shipping silicon on a reasonable timeline. Another part of this story is, as with minion cores, in moving more aspects of the design from fixed hardware to being software configurable.

As to your second point, I agree - open source hardware is no silver bullet for unearthing malicious backdoors. Being able to audit for unintentional issues is useful, but yes - you need to secure or trust your supply chain to know that the chip you have in your hands matches the open RTL.