While they certainly can vary, I think there are enough examples of games done well that writing them off isn't justified. While many of us would absolutely love MORE of the actual thing we loved in the first place, that's not always possible. There will never be a Baldur's Gate III, a KotOR 3, a Halflife 3, a Dark Souls 4, a Neverwinter Nights 3, a Dragon Age: Origins 2, an Alpha Protocol 2, a Vampire Bloodlines 2...oh wait!
Perfect Dark
The first "spiritual successor" I could think of, managed to capture some of the magic of what made Goldeneye 64 so damned good, but repackage it without James Bond. Laptop guns, sci fi setting, Joanna Dark was a great character and quite the coup for Rare.
Pillars of Eternity
Baldur's Gate II has set a bar so high it's probably not fair anymore to compare games to it. It was lightning in a bottle that existed at a time when games could be complex, have more content than any player would see in one playthru, have challenge, put storytelling first and flesh out the world so thoroughly. Pillars wasn't D&D, but it gave us the isometric, party-based combat we no longer get in action RPGs. Being a party, rather than one character, offers a level of strategy and complexity that can't be found elsewhere. But some players don't like managing one inventory, let alone 4-6, and that's fair enough.
Torment: Tides of Nemeneria
So they couldn't use the Planescape setting, but Torment was remember for it's exceptional storytelling, characterisation and the reveals surrounding the protagonist (in particular, his amnesia, immortality, past "lives" and relationships to the NPCs in the game) and cool as Planescape was, it wasn't the important part.
BioShock/Deus Ex/Thief/Prey 2017/Dishonored
The "Shock like" or "immersive sim" made most famous by System Shock 2 didn't just spawn spiritual successors, but a whole genre. Some of these are franchises themselves, trilogies in some cases (the Thief reboot didn't happen). BioShock itself now has "spiritual successors" of its own (Atomic Heart and/or Close to the Sun). Thief created a whole new paradigm of the Sneak 'em Up for "First Person Sneaker"). Deus Ex is widely regarded as one of the greatest PC games of all time and itself refined what System Shock 2 and made it better. Threads could be dedicated to this entire category and each entry within it, so I'll move on.
Dragon Age: Origins
Old BioWare's own spiritual successor to Baldur's Gate II has mixed opinions. It was a dark world of racism and violence, elves weren't the graceful forest beings we came to know them as, but gutter rats. There was no option for a 100% happy ending. But, it had great and memorable NPCs, simplified but quite satisfying party combat, real-time pause combat, a well-told story with the freedom to approach the game in many ways. The Origins were a unique and utterly brilliant feature sorely missed in subsequent games. That we'll never get a sequel to this game is a tragedy, since neither the so called "2" or Inquisition were related except for cameos and references. It would also be the last time we'd get a BioWare game *without* a fully voiced protagonist and all the downsides that comes with.
Assassins Creed
This is a tough one. I could write an essay on this subject, but the core of what would become Assassins Creed came about because of the success of the Prince of Persia "Sands" trilogy. The PoP games were masterpieces, with the magic of level design, mechanics, visuals, sound, characters, storytelling all beautifully woven together to make games, while not individually perfect perhaps, worthy of inclusion in any "Greatest of all time" list of games. The most memorable parts of the game beyond the arrogant Prince and the Sands of course, are the parkour and combat parts.
Ubisoft wanted an annual Call of Duty thing, and AssCreed was the result of taking the parkour and combat elements from PoP and putting it into a sandbox with just enough story to explain the need for gameplay. The never-ending annual releases however means it will never tell a real story (Ezio probably came closest to having one) or ever end. But Ubisoft took the core of PoP and turned it into a sandbox parkour thing for the masses, and it seems to be working.
CoH Successor Projects
My last entry, City of Heroes untimely passing has left a hole that cannot be filled in the hearts of a lot of gamers around the world. Despite being profitable, NCSoft sunset it and left many of us homeless and unsure what to do in a world where we couldn't fire up the game. City of Titans was kickstarter by fans who were game devs to create a spiritual successor, since Cryptic Studios own attempt, Champions Online was...well...crap. Some CoT creatives split off because of creative differences to create Heroes and Villains. Next, Valiance Online was announced, also a spiritual successor. This is the first of them to reach Alpha and also the only one i've crowdfunded. Lastly, out of nowhere came the announcement of Ship of Heroes, a fourth spiritual successor with a unique sci fi twist. I've kept an eye on them all over the years but they do seem stuck in perpetual development. I am also afraid that the age of the MMO has now passed, that even the love and nostalgia for CoH may not be enough to help these games succeed.
I would also add other games like the myriad Diablo successors (Path of Exile, Torchlight), Fallout New Vegas (a spiritual successor to its own franchise, specifically the original Fallout games made by Obsidian members formerly part of Interplay/Black Isle), Dark Souls (successor to Demons Souls) and the controversial Star Citizen, ostensibly pitched as a spiritual successor to Freelancer and Wing Commander but is already 5 years overdue, still in Alpha, and ran out of money despite being the most successful crowdfunded game ever.
