OK, obviously you didn't even try to understand what I said about "creative packaging." So I'm gonna draw it in (digital) crayon and show you the problem.
Diagrams on top are a sample portable amp. No DAC circuit occupying one side of the battery running from rear panel USB to the amp circuit in front. You made the assertion that 10% thicker chassis with all other components the same size will give you a large enough battery to get 50% more battery life, which, we should assume for simplicity's sake, would need roughly a 50% larger battery.
Amp to the right is roughly 30% thicker (black box) because this is a PITA to draw. All circuit components (red - PCB, pot, diode/chip heatsink, and power cap visible) are the same size. Original battery size (blue) is slightly enlarged (purple), but
there is unused space (teal) above the components red) that remained the same size. You are not accounting for how the battery is a single cell unit to keep parts and assembly costs down. At best, a 30% thicker chassis is needed to get you a roughly 30% larger battery for roughly 30% more battery life. And that assumes that manufacturers will find a battery that exact size rather than leave space around it, or design the amp in such a way as to maximize space around such a larger battery.
Amps are
not made with the same costs and volume sales to recoup investments as laptops (bottom diagrams). More traditional 12in workstation "ultraportable" laptop from a few years ago on the left has a wide, thick, but otherwise not too large battery to the rear, with the motherboard (red) to the front and there is a cooling fan for the CPU heatpipe (light blue), and an optical drive to the right (yellow), and an HDD sandwiched between them. On the right is a Core M laptop like the Macbook 12in and hybrid Lenovo Yoga series powered by the same kind of processor (Yoga is used even for more traditional laptops as long as it is also has a hybrid tablet mode). Motherboard (red) is wider but shorter front to rear, has an M.2 drive (green) which along with the absence of an optical drive, saves space. These newer laptops depending on packaging might have a large battery pack with two or three cells to fill up the space around the motherboard (Macbook 12 mobo is actually outflanked by battery cells). Multi-cell battery packs aren't as standard as you'd think, and apart from some portable amps that basically used several AA or AAA batteries bundled together, isn't really a thing that's feasible on these products.
Bottom line: even without accounting for actual volume (which will actually net you less gains), a thicker chassis isn't going to get you as much battery life as you think. Not to mention that your 10% = 50% more battery life is physically impossible given the same battery technology, as well as how, if your starting battery life is 8hours, "15 to 24 hours" isn't exactly "50% more."
You also missed another point. Given the proportions discussed above, and the reality that people nowadays
tend to have a powerbank for their phone anyway, charging the Q5 from it rather than dealing with the increased size of the Q5 makes sense to many. Instead of having a more realistic 50% thicker amp for 50% more battery life you can just have a slim 10500mAh powerbank to charge your phone and the Q5
only when you need it, rather than have the Q5 permanently that thick. Yes, phones can have longer battery life if they weren't squeezing too slim batteries in them, doesn't mean that a 50% thicker phone will net you 50% more battery life.
Actually good quality lithium batteries will last longer. Cahre cycles number in the thousands if not tens of thousands. What actually increases wear is heat, not simply from tightly packed components, but on how you recharge. If you always recharge while the device is in use, then that's a problem since it puts a continuous drain-charge-drain cycle, and it's not simply from that action alone but due to heat build up in the process. Too fast chargers also have a similar though no longer as severe impact (ie not like with NiMH batteries). In short, switch it off, charge it as often as you can and keep the battery full. Even if you have to sleep and leave it plugged in since presumably it has overcharge protection and if the amp is off then there won't be continuous cycling.
If anything the more real problem is whether Fiio will maintain a supply of the batteries. Even if the batteries were larger but you charge while in use you'd still wear it out sooner
and end up neeeding a replacement anyway. This is a bigger concern than using a bigger battery unless we're talking about a really short battery life on a single charge, like the Lisa amps.