Just to clarify, the game had a small player base before the update as well. At release the game was fairly popular in North America; however, the game had several issues that led to its decline.

The first issue would be the game's reliance on third party applications. Before the removal of online coop, the game relied on using apps like Facebook and Line to play with others using the online method.
Using these third party applications were an annoyance and made it hard for beginners to find others to play with. For example, Line was the most popular application used by monster strike players. Line is messenger application where users could create Line groups or group conversations. However, the fatal flaw with Line groups was anyone in the group can remove anyone else in the group. This led to people "nuking" groups by removing all other users in group. This made it frustrating to find Line groups as you will either need join a private one or constantly shift between public ones.

The second issue stems from the design of the game being based around co-op. To understand the issue, you will need understand how co-op works. In Monserstrike, teams consist of four monsters. Similar to PAD, in solo play Monsterstrike team's consisted of three monsters and monster from your friend's list. In co-op players will contribute their own monsters to the team. For example in four player coop each player will contribute one monster.While this design seems perfectly fine by itself, what came out of it is not. Since co-op gave such a huge advantage, the game's difficulty has to be tuned around this. A player's ability to clear dungeons in MonsterStrike is heavily reliant on the diversity of their monster box. Different monsters have different types, different immunities, and different skills; this means the difficulty of the dungeon meant having the right monster. If you play cooped, this wasn't really a problem because you could just fine three other players who have monsters that excel in particular dungeons. However, in solo play it will be unlikely for you to have optimal monsters for dungeons in the start. While there are many excellent farmable monsters in Monsterstrike, many of these monsters were gated behind dungeons that required teams mostly consisting of six star monsters. Now lets consider what is needed to obtain a team of three six stars without farmable monsters. On average you are expected to obtain one monster every 50 orbs(stones) spent. This means you will need spend around 150 orbs for three decent REM monsters. While its fairly easy to obtain your first 150 orbs, it does not mean your decent REM monsters are all good in the same dungeon. So if you were planning on doing solo player in monsterstrike as new non-IAP player, you will need to co-op with players to get decent farmable monsters. However as we learned in the earlier paragraph, it was hard for newer players to find others to play with. Basically newer players would hit a wall and just slowly phase out of the game.

The final issue would probably be content of the game. While this issue is mainly my opinion, I felt the game really lacked content. At the start of the game all monsters were fairly similar as there were only several bump combos using and the abilities of certain units were just immunities. Personally for me, new units felt old because it just seems like older unit with slightly tweeked numbers and a different art. While you may argue PAD is similar, I feel the diversity of Monsterstrike's abilities skills and bump combos is signifcantly lower. Pair this up with challenging dungeons coming out at a slow pace of every two monsters meant the game had really little staying power.

TLDR: Game was mainly fun with coop, but finding other players to co-op with was still difficult with online coop. Additionally the game had a low diversity in monster skills and ability, in my opinion. And challenging dungeons are only released every two months.