Granblue Fantasy is not really Niche in Japan or in parts of Asia, it is a multi billion generating Mobile IP.
Tekken 8 isn't really a useful comparison, that series has been dead in Japan for a long long time, as is the case with pretty much all other Fighting Game Series not called Street Fighter(and even then, it's not selling a lot on PS).
Point of Comparison for Granblue Fantasy compared to other JRPGs releasing around the same time are the better data points.
Persona 3 Reload/Like a Dragon 8
Famitsu Sales: Week 5, 2024 (Jan 29 - Feb 04)
02./00. [PS5] Granblue Fantasy Relink # <RPG> (Cygames) {2024.02.01} (¥7.980) -
48.754 / NEW <80-100%>
05./00. [PS4] Granblue Fantasy Relink # <RPG> (Cygames) {2024.02.01} (¥7.980) -
22.083 / NEW <80-100%>
01./00. [PS5] Persona 3 Reload # <RPG> (Atlus) {2024.02.02} (¥8.800) -
76.368 / NEW <60-80%>
03./00. [PS4] Persona 3 Reload # <RPG> (Atlus) {2024.02.02} (¥8.800) -
40.024 / NEW <60-80%>
Famitsu Sales: Week 6, 2024 (Feb 05 - Feb 11)
05./02. [PS5] Granblue Fantasy Relink # <RPG> (Cygames) {2024.02.01} (¥7.980) - 8.448 /
57.202 <80-100%> (-83%)
16./05. [PS4] Granblue Fantasy Relink # <RPG> (Cygames) {2024.02.01} (¥7.980) - 3.855 /
25.938 <80-100%>
(-83%)
04./01. [PS5] Persona 3 Reload # <RPG> (Atlus) {2024.02.02} (¥8.800) - 9.686 /
86.054 <60-80%> (-87%)
10./03. [PS4] Persona 3 Reload # <RPG> (Atlus) {2024.02.02} (¥8.800) - 6.811 /
46.835 <60-80%>
(-83%)
Like a Dragon 8 came out a week earlier and did these numbers:
Famitsu Sales: Week 4, 2024 (Jan 22 - Jan 28)
01./00. [PS5] Like a Dragon: Infinite Wealth <RPG> (Sega) {2024.01.26} (¥8.800) -
102.940 / NEW <60-80%>
03./00. [PS4] Like a Dragon: Infinite Wealth <RPG> (Sega) {2024.01.26} (¥8.800) -
77.134 / NEW <60-80%>
Famitsu Sales: Week 5, 2024 (Jan 29 - Feb 04)
07./03. [PS4] Like a Dragon: Infinite Wealth <RPG> (Sega) {2024.01.26} (¥8.800) - 16.503 /
93.637 <60-80%>
(-79%)
08./01. [PS5] Like a Dragon: Infinite Wealth <RPG> (Sega) {2024.01.26} (¥8.800) - 15.388 /
118.328 <60-80%> (-85%)
As we can see GBF did considerally worse than LaD, and even was outdone by P3R which is a remake.
I don't think it's a failure, but nor is it really a big success in Japan on PS5.
It's just okay when we consider software sales of PS5 in general within the region.
BGF was outperform even worse in EU/NA, but that's to be expected, LaD and Persona have been growing the brand globally compared to Granblue which has remained a JP/China centric with a few more Asia countries as well.
GBF is actually one of the few games that has benefited tremendously from a PC version due to the IP being very popular in China, it would've struggled to get 1M copies worlwide without the PC version.
If we look at the reviews breakdown by language, the numbers are very very Chinese language speaking.
If you add both Chinese language(Simple/Traditional) it's over 60% of reviews in Chinese, this is pretty rare for most releases.
I can assure you that many many Japanese publishers are expecting huge software drops in Japan due to the sales trend in the region, pretty much every franchise has cratered even accounting for the rise of digital.
PS3 ----> PS4 ---> PS5 are seeing software drops for pretty much all games, and the trend is not going to reverse anytime soon, it's just the reality of the market.
Third party publishers either need a Switch version if they want to see increased local sales or they're just going to have to multiplatform with competent PC ports if they want to grow worldwide sales.