I think actually guys like Evans and Sastre are relevant, because they were, as Froome is, GT specialists who performed at a very consistent level for a number of years before tailing off after 34. No, their best might not have been as good as Froome's best, but the consistency they showed (and in Evans' case, the relative late blooming - who knows what he might have achieved had he been racing with his post-Mendrisio mindset from 2006 onward). Purito and Valverde are harder to say are relevant, but they account for a lot of those older rider podiums - Purito didn't get much freedom to lead until he was already 30 and Valverde missed two years of career in the middle of it. Horner is an outlier, we all know that, but there is something to be said for the fact that even somebody who really, really should be relevant in the discussion of Froome, Tony Rominger - a massive late bloomer (later age-wise than Froome) who came to the sport late, became a top contender even later, and won four GTs in the era of Indurain, only managed two podiums at an age older than Froome's current age. People like Simoni might be relevant - targeting the Giro in the era of the biggest contenders being all-about-the-Tour and the strength of Saeco's red terror means his Giro performances could be more relevant, the issue being that Froome may reach a stage where he isn't able to contest the win at the Tour anymore but can at the Vuelta because, as we all know, Froome performs at his best when it's very hot, he loves the Vuelta, and its mid-length but very steep climbs suit his yo-yoing style on bad days, and he can put people to the sword on his good days, and of course while it has redressed the balance massively from its days in April, the field will always be stronger at the Tour because it's the sole focus of every rider on the startline, plenty at the Vuelta will either focus on the Worlds, use the Worlds as an 'out' if they don't have the form, or you'll see teams send a secondary leader if their bonanza leader does the Tour and calls an end to their season.
Armstrong wasn't too far from winning on his comeback, but only because the parcours was designed with his comeback in mind. Yes, he was good enough to remain in contention anyway, but let's not pretend that he didn't get put comfortably out of the reckoning in the only proper mountain stage of the race. While it obviously wasn't his primary target of the season, a more realistic assessment of Armstrong's comeback level was that year's Giro. He had two shows of strength in comeback 2.0 - one in the Giro and one in the Tour, when he rode across to attacking GC men with pace and ferocity. In the Giro, he got across to the group with Sastre, di Luca, Menchov and somebody else - possibly Garzelli? - but the second he arrived, Sastre attacked again and dropped Lance like a stone (possibly retribution for the disrespect Lance showed him in basically saying, almost in these words, that he was coming back because if a guy like Sastre can win the Tour and Vande Velde can come 4th, it's easy). In the Tour, he was dropped by Contador and the Schlecks in the lead group on the Petit Saint-Bernard, and he stomped on the pedals and rode impressively across to them. Then there was an immediate ceasefire, about 40 people ended up in the group and they all waited for Dave freaking Zabriskie to lead them down the descent into Bourg Saint-Maurice.