I've wondered why they didn't go that route (OK, it would have been more mass, and the refurbishing cost would have been high, although perhaps not as high as OTL's fragile tiles).
The nitty-gritty of it is that they thought that too in the early '60s, so when X-15A-2 was rebuilt following a minor accident, they went to the trouble of applying a Martin-designed ablator coating to test the theory, in addition to adding drop tanks and a dummy scramjet. In theory, X-15A-2 was able to go faster and higher following its rebuild than any of the other X-15s. In practice, it got up to Mach 6.33 just fine before the ablator was applied, but the highest-speed ablator test flight very nearly led to the vehicle's failure, as the scramjet interacted in very unexpected ways with the rest of the structure. Even in areas where the scramjet had not had an impact, though, the ablator was highly ineffective, it actually made heating worse in some areas, and it was badly damaged. This was after nearly 6 weeks of work to apply it to the vehicle, which of course was a lot smaller than an orbiter. Even with the problems associated with the tiles, orbiters usually only took 12-16 weeks to turnaround, so this would probably have made things go much slower.
In the event, X-15A-2 never flew again, and is now on display at Wright-Patterson.
Do you have a cite for that? I'd like to look it up.
Edit: I've always thought that ablatives on a thin alumin(i)um shoe would allow for easy reusability (slip the shoe off, put another on), although at the cost of some weight.
See NASA SP-2007-4232,
Facing the Heat Barrier: A History of Hypersonics, or NASA SP-2000-4518,
Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane for more details about the trouble with X-15A-2. Both are easily obtainable for free in pdf form from the NASA History Office.
It wouldn't be all that difficult to make it happen. It's a matter of cost, not technology. We can already get to Mars, the problem is finding contractors who won't gouge you for ten years' worth of "research", or raise the final price tag ten-fold over what they promised. Oh, and skip that thirty year long detour known as the space shuttle program too.
This is basically the same as saying "well, Sealion isn't that difficult to make happen, you just need Germany to invest everything in a navy for thirty years prior." Sure, but that totally ignores context. Germany can't invest everything in a navy for thirty years prior to launching an invasion of Britain, and NASA doesn't have the political support to get the huge amounts of money needed to launch something like the 90-day report, Integrated Project Plan, or Project Constellation, all of which are essentially what the OP is asking for (the details may be different...but the overall cost is likely to remain more or less similar, simply because any such plan is going to need to be large and complicated).
Technically speaking as well, hindsight is a wonderful thing. We know
now that the space shuttle, as designed and built, was not very good...but they didn't know that
then. And
then they thought that it would be a very good thing indeed, they thought they were going to get Skylon or at least Falcon 9 limit performance out of the Shuttle. In which case it would have been a great idea.
There are also significant advantages we have now that they didn't have then, more than a decade of long-duration spaceflight for example, or much more exploration of Mars in much more depth. In both cases this has led to an appreciation of issues and risks which they just didn't know existed in the 1960s and 1970s, like the enormous difficulty of Martian entry, descent, and landing operations, or the possibility of long-duration microgravity exposure causing blindness. This means that even the best funded plan is much more likely to fail in the 1970s than today, simply because they won't take countermeasures against threats they don't know exist.