As long as Lapua and Norma are in business, there will be genuine properly dimensioned very consistent and very strong 6.5X55mm brass on sale. Both companies' business was founded on this cartridge amongst a few others (such as 7.62X54r for Lapua). I don't see either likely to pack up or go out of business in the near future - their output continually grows whilst Winchester for instance is in decline. You'd be surprised at how much 'US headstamp' brass is made under contract by Norma.
Factory ammo is painfully underloaded thanks to the Norwegian Krag. 20 rounds of Hornady 140gn PSP I used in an F-Class rifle produced 120 fps ES, sooted cases and I back-calculated PMax to around 35,000 psi using QuickLOAD afterwards. Even Lapua's 140gn Scenar Match loading is restricted to 45,000 psi or so, in other words a sensible ceiling for the M1896 Swedish Mauser service rifle. The modern 6.5X55mm is the 6.5X55 SKAN as defined by the European equivalent to SAAMI, the CIP. Its allowed MAP is 55,000 psi and is for use in modern rifles. That's a modest pressure compared to the 60,000-63,000 psi seen in other modern cartridges like 308 Win, 6.5 Creedmoor and 6.5X47 Lapua, but isn't actually that far short of the 58,000 psi they're actually loaded to by the factories. In handloading terms, Lapua brass will certainly take as much pressure and last as long (or as short a time) as more modern cartridges such as the Creedmoor, and like them it's slackening primer pockets that kills a case.
In load development with a Savage Model 12 F-Class rebarrelled to 6.5X55mm from 6.5-284 with a Bartlein 30-inch 5R otherwise factory standard and using a standard no fancy spec PT&G reamer, I did a bit of load development with the 139gn Lapua Scenar a few years back before the Savage PTA action was cannibilised for a 308 FTR build that I needed more urgently. With Viht N160 I got the 139 to 2,999 fps without hard extraction or a ruined case, but silly hot and not one to carry on with. Group size had been optimised 1.5gn and 100 fps or more lower. I settled on Viht N165 and a charge which as it turns out was right on what Lapua lists for this powder and the 139 as its maximum SKAN load. It gave 2,815 or thereabouts MV, a small ES and grouped into 0.2" for five shots at 100 yards. I only had the chance to shoot it in a couple of F-Class matches, one short range, one at 800 before stealing the action, but it did very well. As others have posted, there are plenty of slower burning powders than work well in the cartridge with 140gn class bullets. (We use less and less H4831 in the UK as its availability has become so unreliable and the 284 / Shehane / 7WSM guys snap it up as soon as our importer gets a few hundred tubs in.)
6.5X55AI was reasonably popular in the UK in the very early days of F-Class, but nobody makes off the shelf dies and as soon as 6.5-284 got going few people stuck with it. Now of course hardly anybody shoots 6.5-284 in long-range F either as the sevens rule all. There is an argument that the AI is the best of the 6.5s, the '55 being a little short of case capacity, the '284 having a bit too much and the AI in between is Goldilocks' porridge. Personally, I'd say with today's powders, both the '55 and '284 are superb cartridges and it comes down to how you want to play the perfomance v barrel life equation. I've just had a rifle rebarrelled to 6.5-284 for L-R BR and the occasional mid-range F match and I don't intend to load it 'hot'. The 6.5X55 Bartlein is in store waiting for the Savage 12 action (now in a Dolphin Gun Co. F-Class chassis stock) to become available for it. I also shoot 6.5 Creedmoor and 260 Rem and like both, but have given up on 6.5X47L for all sorts of reasons ... pray that Erik Kortina doesn't read this!