Comment on List of Biggest Dinosaurs by Paleo-King

That is a bit complex, but there have been times when more basal species and lineages actually did better than their derived cousins (i.e. Fusuisaurus was living long after Brachiosaurus was gone). It's not always a matter of out-competing, often when there is a mass-extinction it's more about simply weathering the disaster with fewer losses. An extreme case is the K-T extinction, which wiped out most dinosaurs but spared most crocodiles. And of course crocs are so basal that they're cold-blooded and basically regurgitating the same body plans over and over, and never out-competed dinosaurs on anything like equal terms. Even after the extinction, they didn't exactly dominate the land, but they sure did survive. Yes they have a niche too, but they're not picky, and they need a lot less food. Crocs will eat any sort of meat, and often fast for months after a kill.

On a smaller-scale extinction like the LJ-EK boundary, which killed off the true diplodocids but plenty of dinosaurs and other warm-bloods still made it through, it was the more "all-purpose" types that survived. Dicraeosaurs with their rounded mouths had more diverse feeding options than true diplodocids, so less risk of starving to death in case one food source is wiped out. Early rebbachisaurs (before they became vacuum-mouths) were probably much the same. Brachiosaurs and basal somphospondyls were more derived than any diplodocoids, but they did well because they had rounded mouths full of teeth, plus much more neck range than dicraeosaurs, so literally they could eat anything with leaves. We don't know when the very first titanosaurs appeared, but likely they also had a similar mouth shape, as Malawisaurus retains a basal oval mouth with well-distributed teeth. So in this case it isn't so much about who is "basal" or "derived" but rather who is better suited to a wide range of feeding and habitat options. If the K-T extinction had been less severe, more like the LJ-EK one where many big dinosaurs survived, it's likely that basal and intermediate titanosaurs (wide variety of feeding options) would have dominated and dedicated low-grazing saltasaurs would have been in huge trouble.

The best explanation is that features or sets of features that we label "basal" and "derived" don't necessarily equal "outdated" and "updated". The basal offshoots are often (but not always) less specialized than the derived forms in a clade, which means that they don't do as well in times of high biodiversity and niche specialization, but if there's a big extinction, usually the more over-specialized animals die out first. This means usually the derived members of a clade are the most vulnerable to a mass-extinction, and the basal ones suddenly have an edge.

In normal circumstances the derived ones dominate, but when a mass-extinction occurs it's actually often better to be a generalist "basal" species. So it's likely that rebbachisaurs and dicraeosaurs didn't out-compete diplodocids so much as they were more adaptable to the LJ-EK mass-extinction (i.e. less specialized feeders that could eat more types of plants - keep in mind the hyper-specialized "vacuum mouth" rebbachisaurs like Nigersaurus came millions of years later).

Interestingly, the meat eaters had the opposite happen: ceratosaurs are considered more basal than allosaurs, but most ceratosaurs died out in the LJ-EK extinction because they were the ones that got too specialized (extremely oversized teeth, much more front-heavy, highly reduced arms) whereas allosaurs were more derived in their internal engineering, but retained a pretty general-purpose predatory tool kit (big arms and hand claws, smaller teeth, more flexible jaw joints). Thus Allosaroids dominated the immediate aftermath of the the extinction, with Acrocanthosaurus and the carcharodontosaurs and neovenatorids being top predators. Ceratosauroids made a comeback later as abelisaurids and noasaurids, but these were nearly always 2nd-tier or 3rd-tier predators (unless you're in India or Madagascar).