The problem with the slow and steady approach is that it is just as susceptible to budget slashing as the BOOM-Apollo approach without the awesome result of Apollo. I personally would have the BOOM-Apollo approach followed by a slow and steady use of the huge infrastructure and technology base built up in the boom period, perhaps like a gold rush city or the railway boom infrastructure.
Is that really the case? I'm not sure I fully buy that line of argument. I think your gold rush analogy is apt but in a different way: for every modern thriving city there's a lot more ghost towns.
You say slow-and-steady is just as susceptible to budget cuts but I don't buy it. The difference between a "spectacular heavy lift" based and a "evolutionary medium-lift based" space program is
precisely it's resistance to defunding.
The infrastructure for heavy-lift like the Saturn V or STS is a fixed cost. The workforce is a fixed (and huge) cost. You have have to run LC39 with the same bottom line if you fly ten shuttle flights a year or two. This is one of the big problems I see with SLS, and with exploration-class heavy lift in general: you just don't get the flight rates to see any savings or economies of scale. How many lunar-class heavy-lift rockets have
ever flown? Fifteen Saturn V's and two Energias? Do these launchers and their associated (colossally expensive) infrastructure have
any use or rationale other than expensive exploration or on-orbit megaprojects?
When budgets knives come out (as they did OTL in the 70's) the pure science and exploration stuff will get the axe, with space travel pared down to military use and commercial satellite launch. The Space Shuttle got the funding it did only because a lot of really crazy promises were made about how cheap it would make military and commercial satellite launch. If you have a space program with an Apollo-and-BOOM model you're just shit out of luck because your infrastructure and space travel architecture has no other way to justify itself. Even Saturn 1B was a pretty expensive way of getting three dudes to a space station.
Now, in a slow-and-steady program, with alt-Gemini dragging on, a launch every few months through the sixties, the space race in a languid slow-mo, US launch systems begin to gravitate around some sort of Titan-GLV (or equivalent optimized for civilian roles). If by the late 60's there's the option of some sort of multibody Titan (say, three cores ganged together) you can play legos with cores and SRB's and evolve a pretty wide range of launch capabilities that use the same production facilities, production and handling workforce, and pad facilities. Youre putting spysats in polar orbit, sending men up in evolved Gemini-analogs, sending up MOL-derived civilian station modules, and putting commercial satellites into space with essentially the same rocket.
I'm not pulling this rationale out of nowhere: it's essentially the
Eyes Turned Skywards model at work from the get-go. Instead of going for a modular/refined Saturn capability post-Apollo the US- NASA and USAF- goes for common, scaleable Titan-analog capability post Mercury.
In this sort of "ideal" pacing, if times get rough budget wise you fly fewer guys or wind up with station modules in storage like what happened with the Russian DOS-8 and Pirs modules in the 90's. Things get lean but you don't loose your entire manned launch capability in one fell swoop when Gojira Proxmire stomps through Canaveral and takes away your moon rockets and space planes. Hell, if you refine the rocket design enough (which would probably happen with a common rocket family) you can pinch your pennies and buy capability "on installment" over time until you've saved up enough parts in storage to launch your five-core mega-alt-Titan/Centaur to carry a station core or transstage/lander to TLI.
