You're not forced to power-up the Steem, so the comparison to Bitcoin is like comparing apples with oranges. You can hodl Steem in your wallet, same as Bitcoin

If you don't power up, you incur the full brunt of 8.5% inflation, which is going to be a non-starter for most investors/holders.

That's not the case with Bitcoin (current inflation rate is about 3% and set to drop in half soon), nor with most other newer tokens and coins which have moved away from high inflation models. The few that haven't (Zcash being one well-known example) have been poorly received.

luxury of unstacking it within a shorter period of time, but with less APR

Realistically that means incurring more inflation and won't be attractive. There is no free lunch; any rewards paid to one group (such as people who lock up longer) has a flip side of higher inflation for another group (people who don't), and if the inflation is too high becomes and deterrent to invest.

One way to make the option of holding liquid Steem more attractive ("like Bitcoin") as you suggest would be to greatly reduce inflation (including both rewards paid to content and rewards paid to those who do lock up). Obviously both of those reductions are going to be unpopular with someone, but that doesn't mean they could never happen either.