'NotBack you are going of the deep end a bit here.

The difficulty stratosphere attack you've described doesn't work very well against Monero if you work out actual numbers.

If you wanted to drive up the difficulty 1000x then you would need something approaching a billion typical (old, insecure, low powered) botnet nodes. If you wanted to use higher performance computers say from cloud computing you'd need 10 million or so, which is good portion of the capacity of the big cloud computing vendors. To rent that you would have to displace most or all of their other paying customers (or in the case of vendors such as Amazon or Google, their own usage). That won't happen.

Now lets say you did manage to, somehow, drive the difficulty up 1000x. You would drive the average block time from 2 minutes to 2000 minutes which is around a day and a half. The chain would not completely stall, it would continue to generate blocks at this slow rate. Those blocks would feed into the difficulty adjustment and after a few days the block time would rapidly begin to come down. It would still be slow for quite a while, but the severity would subside. Meanwhile, the blocks would be full of high-paying transactions and the block size would increase. The network would hobble along until it self-healed.

If you tried the possibly more plausible 100x version instead of the ridiculous 1000x version, then the block time only goes to 200 minutes, which hardly slower than Bitcoin on a bad day (I've personally waited over an hour for a block). Again, block size adjustment would start to kick in and clear the transaction backlog. Over time (hours to days, not years) the block time would start to come back down pretty fast anyway.

This ignores that Monero with a billion market cap would probably have a much higher baseline hash rate, meaning not only is 1000x implausible but 100x would probably be as well. And a 10x difficulty attack is just purely money for basically no purpose.

Nothing about this is specific to Monero's algorithm, which probably isn't even all that good. This sort of attack won't work against any current alts with faster base block times and difficulty adjustment algorithms that have been battle-tested not only by malicious parties but by auto-switching pools which do this form of "attack" automatically and routinely by rapidly moving massive amounts of hash rate between coins.

The attack works much better against Bitcoin-style coins (1st gen alts mostly) that start with a higher block time and that maintain a fixed difficulty for a (reasonably long) cycle. You drive up the difficulty during one cycle and the difficulty never adjusts at all until the end of the next cycle so there is an unacceptable wait for the whole cycle (2016 blocks in the case of many early alts that just copied Bitcoin).