History does repeat itself....time & time again-
Lets see if your Creed's & Lapua's are around in 60 +years to mimic the longevity of the 243Win.
Doubtful, as they're just the current trend.
That's an interesting question. History suggests that success or otherwise of a newly introduced cartridge soon shows up and that heavily influences its long-term prospects. There are apparently five or six categories of response to a new mainstream market design:
the 'lemons' - never sell well; only the original manufacturer produces ammunition and brass and often drops them after a few years of poor sales; no other manufacturer adopts them; few makes and models of rifle are chambered for it and within 10 years, 20 at most, they are quietly are dropped by the 'owning' manufacturer. For example, TC models, 6.5mm and 350 Rem Magnum. Very rarely, a cartridge is revived despite long term indifference when a new or modified role appears - 284 Win in over-SAAMI length form as long-range hunting and match round as well as the basis of many wildcats and the factory 6.5-284 Norma. The 7-30 Waters, a failure in its intended role to revive Winchester's 94AE levergun fortunes, but picked up by niche users in single-shot pistols.
the 'never quite make its'. They do well enough to survive but cannot be described as great successes. The 260 Rem falls into this category - apparently now unloved and uncared for by its creator Remington and only a few alternative loadings from other mainstream ammunition companies. It had a big chance of revival some years ago when sniper competitors and others picked it up, but it has been left to boutique manufacturers such as ABM and Prime Ammo to produce match and tactical rounds. Many hunting-only numbers fall into this category even if they are excellent designs - the 358 Winchester, for example, maybe the WSSMs.
the 'flashes in the pan'. Lots of initial excitement and good sales but before other manufacturers adopt it, downsides become apparent and/or 'a better mousetrap' appears. The 264 Winchester Magnum is the classic example - a great success until word of its barrel-burning propensities got around, but more importantly Remington's 7mm Magnum being introduced a couple of years later destroyed its remaining prospects.
the 'almost instant successes' - 223 Rem, 308 Win, WSMs, some of the 1960s Winchester and Remington Magnums. Even when better alternatives appear, many will stick with them. Most firearms and ammunition / brass manufacturers adopt them within five years of launch. They usually have a long-lasting appeal. (I say 'instant success', but 308 Win took a rather long time to be adopted across the board especially in the USA such was the hold of the .30-06 on the market.) I cannot think of one such in the past that once adopted by nearly all manufacturers and US shooters which fades away after 10 or even 20 years. These numbers become 'standards' - people talk about 308 class cartridges; 30-06 class etc. They are usually multi-role - range and field use.
the 'niche players' - either specialist numbers kept going in custom and rebarreled rifles and through handloading. 6mm BR and variants; 6.5X47 Lapua and suchlike. .... or specialist hunting numbers such as some Weatherby Magnums, 458 Lott etc.
The 6.5 Creedmoor falls into the 'instant success' category whether people are happy about that or not; whether they think it deserves it or not. In fact, it apparently outperforms even 308 and 223 at the same points in their development timelines in terms of adoption by manufacturers, but that is due to increased disposable wealth and changes in the market over the last 50 years. The number of people now producing brass and rifles for it is staggering given it's less than 10 years old. On a historical basis such investment and involvement suggests a long future run. It already falls into the 'standard' category replacing the 260 Rem and 6.5X55mm as the 'standard 6.5', for many younger shooters the only cartridge they even know of in this calibre.