The Metaphysics of Haven, as written, are both sensible and easy enough to grasp, and yet incredibly wonky and not entirely useful in understanding the setting.

One of the ideas I've struggled to keep in mind is the idea that earlier ages were vastly more mythic.  

A dirty secret aspect, to keep the earlier ages more epic feeling, is the idea that while they were more 'mythic' they aren't necessarily more 'powerful', the idea that the ancients could simply use magic to brute force their way through issues that can be solved more comtemporaniously using reason...

An idea of this: If an early hunter wished to find deer, he would spend weeks studying certain magics and rituals, communing with the spirits of Deer and predatory animals, which would gift him with a supernatural knowledge of where to find deer and spells to summon the animals towards him... if not actually to create them from the raw stuff of magic  strictly for the purpose of slaughter.

A 'modern' hunter spends as much time and effort learning the habits and preferences of deer, learning to stay upwind, crafting mating calls and staking out likely spots and is, in the main, just as sucessful as the 'epic' hunter from ages past.  It is not to say that he isn't using magic, far from it, but its a much lesser part of his effort.

When the Gods changed the nature of magic (twice that we know of), it forced the mortal races to learn new ways, less raw less power intensive, to influence the world.  In Haven, all crafts involve magic, all skills are, in their own way, ritual spells in practice... at least in theory.

When you have wizards creating dragon like flying mounts and alchemically brewed 'guns' that can be viewed as magic 'death wands' it may seem that the epic nature of the Titans is less epic. The Titans created 'Dragons' to persecute their wars... and Renbluve has smaller dragons carrying their elite cavalry, where is the difference?  

There is none. Rather, what difference there is consists largely of set dressing.

So then why should players be interested in the relics and mysteries of bygone ages?

First and foremost it is far from obvious that the two are like, particularly if one reads through a setting and hears nothing but praise for these ancient artifacts and cultures, or how such feats can not be replicated today.

Second, there is a difference that does matter.  Much of what can be accomplished with 'modern' magics relies on a hard won understanding of the laws of reality underpinning Haven, and all creations are bound by those laws. The ancients on the other hand could force reality to work in ways they preferred by raw application of magic.  The sheer power available to the Titans, for example, is staggering.  Let us postulate a magic weapon, the 'Blade of Ending Ages', a simple sword that can cleave mountains (raw physical force) and sever souls (more mystical power).

To craft such a device in the 'modern age' would require two seperate machines, neither one of which could be held in one hand. To sever mountains would require an immense blade nearly the size of the mountains themselves, powered by vast elemental engines the size of cities.  To sever souls would require an unholy necrotic engine, smaller than the mountain cutting blade, focused on tearing assunder the soul of a single victim at a time...

Possible, certainly, but hardly convienent.  

Of course, in this example it might be a bit harder than otherwise: the Gods have a vested interest in keeping mountains and souls from being cut in twain, thus have made it harder to do either than it might otherwise be.

I understand and to the same thing in different ways.  
You have the magic actually change through history, while I have a little more of a 'closer to the source' approach, but the effect is the same.  Though my differences are not 'set dressing', in that there actually was a bit ore access in earlier days.

If magic has changed, and casters have to learn new ways, how do the artifacts still work?

This is a less well baked post than I normally do but I've been a bit harried recently.

To answer your question:  A thing created is not so easily uncreated, or something to that effect.

Of course, given the tendency for 'created by magic' stuff to fail spectacularly during the various 'corrections' of the system, its fairly significant that there even are artifacts to be found.

Also, a significant exception to 'magic doesn't work that way anymore' is the 'Elder Sorcery' used by the First Men and the Savants of Irem.  While the methods of recreating it have been lost, the actual developed techniques still work as they originally did.  Of course, Elder Sorcery is also the purest expression of 'raw power over technique' in that it is the literally summoning of raw chaos into reality with only a minimal effort to direct and shape it.