3.5 D&D Archives WotC has quite extensive 3.5 archives online for free you all know that right? This is all FREE, there's little to no point in putting it in DDI. As for a Compendium for 3.5 there are already SEVERAL excellent online searchable SRDs, including the PF SRD, the WotC 3.5 SRD (not searchable really, but there it is...). It would be a bit nicer to have it in a Compendium, but seriously, how many people will pay $6-10 a month for what they already have?
As for actually IMPLEMENTING other editions in a Compendium... It ain't all THAT simple. Creating the application is one thing, that would require a decent amount of work, but could be based on enough of the 4e Compendium to probably make it a doable task in that sense. The bigger task would be milling through 10k pages worth of material for whatever edition was wanting to be supported and entering all the data. Remember too, what you see as 'Compendium' is only the tip of an iceberg. There are all the tools used to enter and manage the data, databases, servers, etc. ALL of that stuff has to be retooled to support the very different rules of different editions. This is very much nontrivial and would require significant outlay of manpower, all taken away from the already stretched 4e tool development and maintenance.
Finally I just don't see how this would be monetized. Nobody is going to subscribe to DDI to get a smattering of support for some earlier edition they like to play. WotC could decide "ok, we'll support 1e" but that's only a small fraction of the fans of older editions. If they only support that one maybe eventually they could make it as supported as 4e is, but short of that nobody that plays 1e still is going to pay for 1e DDI. If 5% of the people that play D&D will be attracted by that 1e DDI that's like 1/5th or 1/10th of the people that the same feature in 4e DDI would appeal to. The subset of people that both play 1e still AND are going to be sucked into playing 4e by being attracted to a 1e DDI is what, 1/2, 1/3, 1/20th? of the 1e playing people. Is that 50 people? 100 people? It ain't a lot. I'm betting putting that effort into a set of hot 4e DDI articles and some MB patches will hands down get more new customers than the ENTIRETY of what they can get from 1e support. The same argument can be made for OD&D, 2e, and BECMI.
I'm sure Mike and the D&D team would LOVE to support all these old versions. Mike clearly likes 1e and has a weak spot in his heart for it. I'm sure other WotC people like older editions too. Heck, plenty of them were TSR people back in the day. Seriously, it is all a matter of what can be viably supported in a business sense.
Now, there could be some creative ways to create a support base for older editions. I just wouldn't do it WITHIN WotC. Find some small company that is interested in OSR type gaming. Make a deal with them. Let them put together PDFs and whatever and then back them up on distributing it, lend them the WotC name, the WotC digital online presence. Let them take the risk, give them a cut of any revenue that comes out of it, and just make sure they do a decent quality job. One of these OSR shops would probably LEAP at the chance to be able to have distribution rights on old edition material. Doing it internal to WotC is probably too expensive, but letting the risk be born by someone else MIGHT make sense. It would be tricky to make it work, but if the resources put in is very small it might be viable.
Really though, considering the amount irrational WotC hate that goes around, I don't know that ANYTHING they can ever do is going to make much difference. The people who love to hate on WotC don't do it for sensible reasons. They do it because they need someone to rag on and WotC makes an ideal Great Satan for them to rail against. Objectively WotC's actions have never been unreasonable or worse than that of any other game company, they're just the big boys in the industry and everyone loves to hate on that. TSR got the same treatment in their day too, people called the NAZIs and every other stupid thing for years and they could do nothing that changed that nonsense. I think it is time for WotC to just realize that trying to make the whole community love them is a pipe dream and chasing that phantom is just throwing money into a pit.