The linear model of time estimates here work well for cathedrals but not necessarily for scientific and engineering advancements. For a Cathedral, you have a g

The linear model of time estimates here work well for cathedrals but not necessarily for scientific and engineering advancements. For a Cathedral, you have a general idea of how long it's going to take (at least finite) and know that every piece of material added onto the structure is going to move it closer to the end goal. For things like fusion, anything beyond say 20 years is basically a bullsh_t speculative guess that sounds better than "we don't really know whether this will even work out in the end". It doesn't really matter whether it's 30 or 60. The real question is whether the number is finite or infinite.