Yes its really annoying-even i've been facing the same issue as you are.I want to upgrade my ageing corsair cabinet(not that there's anything bad about it per se) with something more modern with rgb bling but unfortunately most new cabinets in the 5-6k range only have 2-3 hdd bays and virtually no 5.25 inch bays-this really puts many users like me at a disadvantage as we need a cabinet that can accommodate atleast 4 hdds and 1 optical drive(ODDs are still pretty handy for taking backups onto dvds,despite what many people say).
i dont think there's any way to increase the no of 3.5 inch bays by adding third party brackets-if you dont need rgb and other such fancy frills then you can go for corsair spec 1 which has 4 hdd bays.
also some cabinets from local brands like circle also have 4-5 hdd bays:
CIRCLE MID TOWER CABINET (ATX) - CC840 WITH TRANSPARENT SIDE PANEL (BLACK)
This is exactly my case, I also have 4hdds, added over time to my current setup. ODD are also required as most of the drivers come on CD, people might say you can get from net, but I install it from CD first and update from net. This way I dont have to worry about compatibility. There is no other use of drives if we get a new cabinet with one or 2 expansion slots.
Having multiple slots also have downside that we can not have less then 6 sata slots in MB, which are only available above Rs 10000.
Some options I found:
Buy Cooler Master K380 Black Best Price in India mdcomputers.in
Buy Antec GX500 Window Black at Lowest Price in India - mdcomputers.in
Buy CORSAIR MID TOWER 230T ORANGE Best Price in India mdcomputers.in
CORSAIR MID TOWER CABINET (ATX) - 200R TRANSPARENT SIDE PANEL (BLACK)
Adapter to convert 3.5" hdd to 5.25" odd bay use:
*www.amazon.in/MagiDeal-Interface-Adapter-Mounting-Computer/dp/B074QQY6ML/ref=sr_1_5?keywords=3.5"+to+5.25"&qid=1560077447&s=electronics&sr=1-5
thanks, yeah, these have 4-5 slots so better then new lineup. I will not prefer K380 as thats pretty old and with only one USB 3.0 in front. guess I need to compromise in some way to get what I need.
Check older series models, they should have full available expansion slots.
I dont think there is point in having older cabinets, if I have to go back, I would continue with my current cooler master.