A few answers
Old World Blew up - literally.
GW decided that Old World wasn't selling well, did a big investment and End Times campaign and blew up the whole setting and replaced with with Age of Sigmar.
This was done during the Kirby Ceo years when
GW "didn't do community feedback" and from what we can gather there were a few choice middle managers that basically caused chaos with the project.
Suffice to say
AoS launched without ANY sensible rules (they had some "Magic the Gathering Unhinged fun rules"); no points, bases changing to round (copy catting
40K) and removed several armies.
Army removal happened in two ways - either fragmenting into tiny armies or total removal (Bretonnia and Tomb Kings were removed within a short span of it starting)
There was loud outcry; crashing sales and whilst it attracted some (esp those who wanted to write their own rules); the upshot was
GW's upper management was reshuffled, some left, some retired and we got the new age of management.
AoS changed vastly from its launch (which wasn't even a 1.0) to the current 2.0. 2.0 introduced proper rules, points, new ideas (Endless Spells) and generally has firmed things up.
Vampires are actually about to have a SUPER time because
GW is updating that model line almost entirely as the next big Age of Sigmar release. It's not yet got a date, but look up "Soulblight" and you should find loads of links and info. Basically most of the vampire line is old finecast and plastics and is being removed and replaced/updated with a brand new release.
As for why round bases this was mostly because
GW saw rank and file not working and, at the time, decided to copycat
40K which was selling really well. Right now the two even bounce ideas off each other rules wise - what works in one often slips over to the other side and vis versa even though they are distinct systems.
AoS is also seemingly getting a bit better balance in terms of faction powers and diversity and the creative side is outstanding. Competitive events often show quite a wide spread of faction variety turning up - though its still
GW rules so there's still issues here and there.
As for what Age of Sigmar is in terms of story its a shift from character to god driven story setting so the tale has loads of mortal heroes, but the main story focuses on the gods. ~Many from the Old World that escaped. Age of Sigmar is technically the 3rd age, preceeded by the Age of Myth and the Age of Chaos - both of which are historical events that were never present for gamers. The Age of Myth is basically the 8 Realms (think of them as giant discworld settings linked with magical portals) being discovered, populated, tamed and generally build up into grand civilizations. Then Chaos seeped its way in and launched a huge invasion and took many realms and pushed many of the gods into hiding or madness (or both); creating a 500 year Age of Chaos. Sigmar basically sealed his realm away, built an army of reforged souls (souls taken from heroes who were given new bodies and suited in powerful armour); and then launched a huge counter attack at the start of the Age of Sigmar.
Stormcast, when killed, will be brought back by lightning to the forges to be remade, though each time (including the first) is likely to destroy memories and shatter their mind a bit. So each time they are reforged they lose more of themselves and have that replaced with more of a fevered desire to fight chaos. So they do start to crack and go a bit insane/one track minded.
Despite starting as a clear copy-cat of
Gw's marine release method, they've eased right off that approach and the game now features a lot of diverse factions, each with stories and power behind them. There's still a lot of smaller armies that need bulking up~ (Luminieth just got a big second wave this last weekend that went up for pre-order); and some older ones that need some revamped models (eg Skaven and Seraphon).
GW is getting there though and there's been new things like the Ossiarch Bonereapers and Gloomspite Gits (basically a huge expansion of mushroom loving goblins) .
Automatically Appended Next Post: Also there's a new Warhammer Quest game (adventuring) which features a lot of vampiric units. That's going up on pre-order next weekend. It might well sell out but its a medium term product - ergo for a few years
GW will restock it. They just finished Blackstone Fortress which was the
40K version.
So Cursed City (which has a lot of cross over models with the vampires) will last for a few years and is expected to see several expansions over that time
https://www.warhammer-community.com/2021/03/28/sunday-preview-enter-the-cursed-city/
Also some soulblight (vampire) previews
https://www.warhammer-community.com/2021/03/20/warhammer-preview-online-faith-damnation/